docs: Update RCU cross-references as suggested in doc-guide

The RCU documentation contains old-style cross references which
do not follow the best practices outlined in doc-guide/sphinx.rst.
In addition, some of the cross references use URLs that should be replaced
by pathnames.

Update all of these cross references and adjust the surrounding words.

Summary of changes:

- out-of-date plaintext file names (*.txt) -> *.rst
- references by :ref: tags -> path names of *.rst
* use relative paths to .rst files under the RCU/ subdirectory
* use abs paths of Documentation/xxx for other .rst files
- references by URL under https://www.kernel.org/ -> paths of *.rst
- adjust surrounding words of some of updated references.

Note:
The automarkup.py script interprets references via "*.txt" as if they
were via "*.rst", so the *.txt -> *.rst changes should be regarded as
cleanups rather than bug fixes.

Cc: [e-mail]
Cc: [e-mail]
Tested-by: Bagas Sanjaya <[e-mail]>
Reviewed-by: Steven Rostedt (Google) <[e-mail]>
Signed-off-by: Akira Yokosawa <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

Documentation/RCU/Design/Data-Structures/Data-Structures.rst

···
 and from either dyntick-idle or user mode, so that this counter has an
 even value when the CPU is in dyntick-idle mode or user mode and an odd
 value otherwise. The transitions to/from user mode need to be counted
-for user mode adaptive-ticks support (see timers/NO_HZ.txt).
+for user mode adaptive-ticks support (see Documentation/timers/no_hz.rst).
 
 The ``->rcu_need_heavy_qs`` field is used to record the fact that the
 RCU core code would really like to see a quiescent state from the

Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst

···
 period also drove it to completion. This straightforward approach had
 the disadvantage of needing to account for POSIX signals sent to user
 tasks, so more recent implemementations use the Linux kernel's
-`workqueues <https://www.kernel.org/doc/Documentation/core-api/workqueue.rst>`__.
+workqueues (see Documentation/core-api/workqueue.rst).
 
 The requesting task still does counter snapshotting and funnel-lock
 processing, but the task reaching the top of the funnel lock does a

Documentation/RCU/Design/Requirements/Requirements.rst

···
 outermost RCU read-side critical section containing that
 rcu_dereference(), unless protection of the corresponding data
 element has been passed from RCU to some other synchronization
-mechanism, most commonly locking or `reference
-counting <https://www.kernel.org/doc/Documentation/RCU/rcuref.txt>`__.
+mechanism, most commonly locking or reference counting
+(see ../../rcuref.rst).
 
 .. |high-quality implementation of C11 memory_order_consume [PDF]| replace:: high-quality implementation of C11 ``memory_order_consume`` [PDF]
 .. _high-quality implementation of C11 memory_order_consume [PDF]: http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf

Documentation/RCU/arrayRCU.rst

···
 
 Hash tables are often implemented as an array, where each array entry
 has a linked-list hash chain.  Each hash chain can be protected by RCU
-as described in the listRCU.txt document.  This approach also applies
-to other array-of-list situations, such as radix trees.
+as described in listRCU.rst.  This approach also applies to other
+array-of-list situations, such as radix trees.
 
 .. _static_arrays:
 

Documentation/RCU/checklist.rst

···
 		prevents destructive compiler optimizations.  However,
 		with a bit of devious creativity, it is possible to
 		mishandle the return value from rcu_dereference().
-		Please see rcu_dereference.txt in this directory for
-		more information.
+		Please see rcu_dereference.rst for more information.
 
 		The rcu_dereference() primitive is used by the
 		various "_rcu()" list-traversal primitives, such
···
 		primitives.  This is particularly useful in code that
 		is common to readers and updaters.  However, lockdep
 		will complain if you access rcu_dereference() outside
-		of an RCU read-side critical section.  See lockdep.txt
+		of an RCU read-side critical section.  See lockdep.rst
 		to learn what to do about this.
 
 		Of course, neither rcu_dereference() nor the "_rcu()"
···
 	primitives when the update-side lock is held is that doing so
 	can be quite helpful in reducing code bloat when common code is
 	shared between readers and updaters.  Additional primitives
-	are provided for this case, as discussed in lockdep.txt.
+	are provided for this case, as discussed in lockdep.rst.
 
 	One exception to this rule is when data is only ever added to
 	the linked data structure, and is never removed during any
···
 	both rcu_barrier() and synchronize_rcu(), if necessary, using
 	something like workqueues to to execute them concurrently.
 
-	See rcubarrier.txt for more information.
+	See rcubarrier.rst for more information.

Documentation/RCU/rcu.rst

···
 must be long enough that any readers accessing the item being deleted have
 since dropped their references.  For example, an RCU-protected deletion
 from a linked list would first remove the item from the list, wait for
-a grace period to elapse, then free the element.  See the
-:ref:`Documentation/RCU/listRCU.rst <list_rcu_doc>` for more information on
-using RCU with linked lists.
+a grace period to elapse, then free the element.  See listRCU.rst for more
+information on using RCU with linked lists.
 
 Frequently Asked Questions
 --------------------------
···
 - If I am running on a uniprocessor kernel, which can only do one
   thing at a time, why should I wait for a grace period?
 
-  See :ref:`Documentation/RCU/UP.rst <up_doc>` for more information.
+  See UP.rst for more information.
 
 - How can I see where RCU is currently used in the Linux kernel?
 
···
 
 - What guidelines should I follow when writing code that uses RCU?
 
-  See the checklist.txt file in this directory.
+  See checklist.rst.
 
 - Why the name "RCU"?
 
   "RCU" stands for "read-copy update".
-  :ref:`Documentation/RCU/listRCU.rst <list_rcu_doc>` has more information on where
-  this name came from, search for "read-copy update" to find it.
+  listRCU.rst has more information on where this name came from, search
+  for "read-copy update" to find it.
 
 - I hear that RCU is patented?  What is with that?
 

Documentation/RCU/rculist_nulls.rst

···
 protect read-mostly linked lists and
 objects using SLAB_TYPESAFE_BY_RCU allocations.
 
-Please read the basics in Documentation/RCU/listRCU.rst
+Please read the basics in listRCU.rst.
 
 Using 'nulls'
 =============

Documentation/RCU/whatisRCU.rst

···
 	be delayed.  This property results in system resilience in face
 	of denial-of-service attacks.  Code using call_rcu() should limit
 	update rate in order to gain this same sort of resilience.  See
-	checklist.txt for some approaches to limiting the update rate.
+	checklist.rst for some approaches to limiting the update rate.
 
 rcu_assign_pointer()
 ^^^^^^^^^^^^^^^^^^^^
···
 	must prohibit.	The rcu_dereference_protected() variant takes
 	a lockdep expression to indicate which locks must be acquired
 	by the caller. If the indicated protection is not provided,
-	a lockdep splat is emitted.  See Documentation/RCU/Design/Requirements/Requirements.rst
+	a lockdep splat is emitted.  See Design/Requirements/Requirements.rst
 	and the API's code comments for more details and example usage.
 
 .. 	[2] If the list_for_each_entry_rcu() instance might be used by
···
 
 This section shows a simple use of the core RCU API to protect a
 global pointer to a dynamically allocated structure.  More-typical
-uses of RCU may be found in :ref:`listRCU.rst <list_rcu_doc>`,
-:ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst <NMI_rcu_doc>`.
+uses of RCU may be found in listRCU.rst, arrayRCU.rst, and NMI-RCU.rst.
 ::
 
 	struct foo {
···
 	RCU read-side critical sections that might be referencing that
 	data item.
 
-See checklist.txt for additional rules to follow when using RCU.
-And again, more-typical uses of RCU may be found in :ref:`listRCU.rst
-<list_rcu_doc>`, :ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst
-<NMI_rcu_doc>`.
+See checklist.rst for additional rules to follow when using RCU.
+And again, more-typical uses of RCU may be found in listRCU.rst,
+arrayRCU.rst, and NMI-RCU.rst.
 
 .. _4_whatisRCU:
 
···
 
 	kfree_rcu(old_fp, rcu);
 
-Again, see checklist.txt for additional rules governing the use of RCU.
+Again, see checklist.rst for additional rules governing the use of RCU.
 
 .. _5_whatisRCU:
 
···
 promotes synchronize_rcu() to a full memory barrier in compliance with
 the "Memory-Barrier Guarantees" listed in:
 
-	Documentation/RCU/Design/Requirements/Requirements.rst
+	Design/Requirements/Requirements.rst
 
 It is possible to nest rcu_read_lock(), since reader-writer locks may
 be recursively acquired.  Note also that rcu_read_lock() is immune