docs: mm/gup: minor documentation update

Now there are 5 cases. Updated the same.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Souptick Joarder <[e-mail]>
Reviewed-by: John Hubbard <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Kirill A. Shutemov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/core-api/pin_user_pages.rst

···
 pin_user_pages*() APIs are clearly distinct from the get_user_pages*() APIs, so
 that's a natural dividing line, and a good point to make separate wrapper calls.
 In other words, use pin_user_pages*() for DMA-pinned pages, and
-get_user_pages*() for other cases. There are four cases described later on in
+get_user_pages*() for other cases. There are five cases described later on in
 this document, to further clarify that concept.
 
 FOLL_PIN and FOLL_GET are mutually exclusive for a given gup call. However,