docs: mm/gup: Minor documentation update

Now there are 5 cases. Updated the same.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Link: https://lore.kernel.org/r/1592422023-7401-1-git-send-email-jrdr.linux@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>