mm/gup.c: update the documentation

mm/gup.c: update the documentation

This patch is an attempt to update the documentation.

 - Add/ remove extra * based on type of function static/global.

 - Add description for functions and their input arguments.

[akpm@linux-foundation.org: s@/*@/**@]
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1588013630-4497-1-git-send-email-jrdr.linux@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>