docs: admin-guide: move sysctl directory to it

The stuff under sysctl describes /sys interface from userspace
point of view. So, add it to the admin-guide and remove the
:orphan: from its index file.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

CREDITS

···
 N: Rik van Riel
 E: [e-mail]
 W: http://www.surriel.com/
-D: Linux-MM site, Documentation/sysctl/*, swap/mm readaround
+D: Linux-MM site, Documentation/admin-guide/sysctl/*, swap/mm readaround
 D: kswapd fixes, random kernel hacker, rmap VM,
 D: nl.linux.org administrator, minor scheduler additions
 S: Red Hat Boston

Documentation/admin-guide/index.rst

···
    README
    kernel-parameters
    devices
+   sysctl/index
 
 This section describes CPU vulnerabilities and their mitigations.
 

Documentation/admin-guide/kernel-parameters.txt

···
 	numa_zonelist_order= [KNL, BOOT] Select zonelist order for NUMA.
 			'node', 'default' can be specified
 			This can be set from sysctl after boot.
-			See Documentation/sysctl/vm.rst for details.
+			See Documentation/admin-guide/sysctl/vm.rst for details.
 
 	ohci1394_dma=early	[HW] enable debugging via the ohci1394 driver.
 			See Documentation/debugging-via-ohci1394.txt for more

Documentation/admin-guide/mm/index.rst

···
 Linux memory management is a complex system with many configurable
 settings. Most of these settings are available via ``/proc``
 filesystem and can be quired and adjusted using ``sysctl``. These APIs
-are described in Documentation/sysctl/vm.rst and in `man 5 proc`_.
+are described in Documentation/admin-guide/sysctl/vm.rst and in `man 5 proc`_.
 
 .. _man 5 proc: http://man7.org/linux/man-pages/man5/proc.5.html
 

Documentation/admin-guide/mm/ksm.rst

···
 
 If a region of memory must be split into at least one new MADV_MERGEABLE
 or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
-will exceed ``vm.max_map_count`` (see Documentation/sysctl/vm.rst).
+will exceed ``vm.max_map_count`` (see Documentation/admin-guide/sysctl/vm.rst).
 
 Like other madvise calls, they are intended for use on mapped areas of
 the user address space: they will report ENOMEM if the specified range

Documentation/core-api/printk-formats.rst

···
 
 For printing kernel pointers which should be hidden from unprivileged
 users. The behaviour of %pK depends on the kptr_restrict sysctl - see
-Documentation/sysctl/kernel.rst for more details.
+Documentation/admin-guide/sysctl/kernel.rst for more details.
 
 Unmodified Addresses
 --------------------

Documentation/filesystems/proc.txt

···
 This chapter  is  heavily  based  on the documentation included in the pre 2.2
 kernels, and became part of it in version 2.2.1 of the Linux kernel.
 
-Please see: Documentation/sysctl/ directory for descriptions of these
+Please see: Documentation/admin-guide/sysctl/ directory for descriptions of these
 entries.
 
 ------------------------------------------------------------------------------

Documentation/networking/ip-sysctl.txt

···
 
 
 /proc/sys/net/core/*
-	Please see: Documentation/sysctl/net.rst for descriptions of these entries.
+	Please see: Documentation/admin-guide/sysctl/net.rst for descriptions of these entries.
 
 
 /proc/sys/net/unix/*

Documentation/sysctl/index.rst Documentation/admin-guide/sysctl/index.rst

···
-:orphan:
-
 ===========================
 Documentation for /proc/sys
 ===========================

Documentation/vm/unevictable-lru.rst

···
 
 The unevictable LRU can be scanned for compactable regions and the default
 behavior is to do so.  /proc/sys/vm/compact_unevictable_allowed controls
-this behavior (see Documentation/sysctl/vm.rst).  Once scanning of the
+this behavior (see Documentation/admin-guide/sysctl/vm.rst).  Once scanning of the
 unevictable LRU is enabled, the work of compaction is mostly handled by
 the page migration code and the same work flow as described in MIGRATING
 MLOCKED PAGES will apply.

fs/proc/Kconfig

···
 	  interface is through /proc/sys.  If you say Y here a tree of
 	  modifiable sysctl entries will be generated beneath the
           /proc/sys directory. They are explained in the files
-	  in <file:Documentation/sysctl/>.  Note that enabling this
+	  in <file:Documentation/admin-guide/sysctl/>.  Note that enabling this
 	  option will enlarge the kernel by at least 8 KB.
 
 	  As it is generally a good thing, you should say Y here unless

kernel/panic.c

···
 /**
  * print_tainted - return a string to represent the kernel taint state.
  *
- * For individual taint flag meanings, see Documentation/sysctl/kernel.rst
+ * For individual taint flag meanings, see Documentation/admin-guide/sysctl/kernel.rst
  *
  * The string is overwritten by the next call to print_tainted(),
  * but is always NULL terminated.

mm/swap.c

···
 /*
  * This file contains the default values for the operation of the
  * Linux VM subsystem. Fine-tuning documentation can be found in
- * Documentation/sysctl/vm.rst.
+ * Documentation/admin-guide/sysctl/vm.rst.
  * Started 18.12.91
  * Swap aging added 23.2.95, Stephen Tweedie.
  * Buffermem limits added 12.3.98, Rik van Riel.