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.

[willy@infradead.org: fix kerneldoc warning]
  Link: https://lkml.kernel.org/r/20240417015933.453505-1-willy@infradead.org
[akpm@linux-foundation.org: remove newline at EOF]
Link: https://lkml.kernel.org/r/20240416031754.4076917-9-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/filesystems/api-summary.rst

@@ -56,9 +56,6 @@ Other Functions
 .. 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

@@ -50,6 +50,7 @@ filesystem implementations.
 .. toctree::
    :maxdepth: 2
 
+   buffer
    journalling
    fscrypt
    fsverity