MAINTAINERS/MEMORY MANAGEMENT: add document files for mm

Memory managment subsystem documents ('admin-guide/mm/' and 'mm/' under
'Documentation/') are not marked as managed under memory management
subsystem. This makes 'get_maintainer.pl' for changes to the documents
sub-optimal. Mark the documents as part of mm subsystem on MAINTAINERS
file.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: SeongJae Park <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
Acked-by: Mike Rapoport (Microsoft) <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

MAINTAINERS

···
 W:	http://www.linux-mm.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
 T:	quilt git://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new
+F:	Documentation/admin-guide/mm/
+F:	Documentation/mm/
 F:	include/linux/gfp.h
 F:	include/linux/gfp_types.h
 F:	include/linux/memfd.h