mm/gup: documentation fix for pin_user_pages*() APIs

mm/gup: documentation fix for pin_user_pages*() APIs

All of the pin_user_pages*() API calls will cause pages to be
dma-pinned.  As such, they are all suitable for either DMA, RDMA, and/or
Direct IO.

The documentation should say so, but it was instead saying that three of
the API calls were only suitable for Direct IO.  This was discovered
when a reviewer wondered why an API call that specifically recommended
against Case 2 (DMA/RDMA) was being used in a DMA situation [1].

Fix this by simply deleting those claims.  The gup.c comments already
refer to the more extensive Documentation/core-api/pin_user_pages.rst,
which does have the correct guidance.  So let's just write it once,
there.

[1] https://lore.kernel.org/r/20200529074658.GM30374@kadam

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Acked-by: Souptick Joarder <jrdr.linux@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Vlastimil Babka <vbabka@suse.cz>
Link: http://lkml.kernel.org/r/20200529084515.46259-1-jhubbard@nvidia.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>