mm: document follow_page()

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Dan Carpenter <error27@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>