mm: docs: fixup punctuation

so that kernel-doc will properly recognize the parameter and function
descriptions.

Link: http://lkml.kernel.org/r/1516700871-22279-2-git-send-email-rppt@linux.vnet.ibm.com


Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>