doc: convert 'subsection' to 'section' in gfp.h

Patch series "Remove remaining parts of congestion tracking code", v2.

This patch (of 11):

Various DOC: sections in gfp.h have subsection headers (~~~) but the
place where they are included in mm-api.rst does not have section, only
chapters.

So convert to section headers (---) to avoid confusion.  Specifically if
sections are added later in mm-api.rst, an error results.

Link: https://lkml.kernel.org/r/164549971112.9187.16871723439770288255.stgit@noble.brown
Link: https://lkml.kernel.org/r/164549983733.9187.17894407453436115822.stgit@noble.brownSigned-off-by: NeilBrown <neilb@suse.de>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Chao Yu <chao@kernel.org>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <Anna.Schumaker@Netapp.com>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Cc: Lars Ellenberg <lars.ellenberg@linbit.com>
Cc: Paolo Valente <paolo.valente@linaro.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/gfp.h

@@ -79,7 +79,7 @@ struct vm_area_struct;
  * DOC: Page mobility and placement hints
  *
  * Page mobility and placement hints
- * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ * ---------------------------------
  *
  * These flags provide hints about how mobile the page is. Pages with similar
  * mobility are placed within the same pageblocks to minimise problems due
@@ -112,7 +112,7 @@ struct vm_area_struct;
  * DOC: Watermark modifiers
  *
  * Watermark modifiers -- controls access to emergency reserves
- * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ * ------------------------------------------------------------
  *
  * %__GFP_HIGH indicates that the caller is high-priority and that granting
  * the request is necessary before the system can make forward progress.
@@ -144,7 +144,7 @@ struct vm_area_struct;
  * DOC: Reclaim modifiers
  *
  * Reclaim modifiers
- * ~~~~~~~~~~~~~~~~~
+ * -----------------
  * Please note that all the following flags are only applicable to sleepable
  * allocations (e.g. %GFP_NOWAIT and %GFP_ATOMIC will ignore them).
  *
@@ -224,7 +224,7 @@ struct vm_area_struct;
  * DOC: Action modifiers
  *
  * Action modifiers
- * ~~~~~~~~~~~~~~~~
+ * ----------------
  *
  * %__GFP_NOWARN suppresses allocation failure reports.
  *
@@ -256,7 +256,7 @@ struct vm_area_struct;
  * DOC: Useful GFP flag combinations
  *
  * Useful GFP flag combinations
- * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ * ----------------------------
  *
  * Useful GFP flag combinations that are commonly used. It is recommended
  * that subsystems start with one of these combinations and then set/clear