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.brown


Signed-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>