doc: split buffer.rst out of api-summary.rst

Buffer heads are no longer a generic filesystem API but an optional
filesystem support library. Make the documentation structure reflect
that, and include the fine documentation kept in buffer_head.h. We could
give a better overview of what buffer heads are all about, but my
enthusiasm for documenting it is limited.

[[e-mail]: fix kerneldoc warning]
Link: https://lkml.kernel.org/r/[e-mail]
[[e-mail]: remove newline at EOF]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Cc: Pankaj Raghav <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/filesystems/api-summary.rst

···
 .. kernel-doc:: fs/namei.c
    :export:
 
-.. kernel-doc:: fs/buffer.c
-   :export:
-
 .. kernel-doc:: block/bio.c
    :export:
 

Documentation/filesystems/buffer.rst

···
+Buffer Heads
+============
+
+Linux uses buffer heads to maintain state about individual filesystem blocks.
+Buffer heads are deprecated and new filesystems should use iomap instead.
+
+Functions
+---------
+
+.. kernel-doc:: include/linux/buffer_head.h
+.. kernel-doc:: fs/buffer.c
+   :export:

Documentation/filesystems/index.rst

···
 .. toctree::
    :maxdepth: 2
 
+   buffer
    journalling
    fscrypt
    fsverity