mm: add orphaned kernel-doc to the rst files.

There are many files in mm/ that contain kernel-doc which is not
currently published on kernel.org.  Some of it is easily categorisable,
but most of it is going into the miscellaneous documentation section to
be organised later.

Some files aren't ready to be included; they contain documentation with
build errors.  Or they're nommu.c which duplicates documentation from
"real" MMU systems.  Those files are noted with a # mark (although really
anything which isn't a recognised directive would do to prevent inclusion)

Link: https://lkml.kernel.org/r/20230818200630.2719595-5-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>