docs: Catch up with the new location of get_user_pages_fast()

docs: Catch up with the new location of get_user_pages_fast()

Commit 050a9adc6438 ("mm: consolidate the get_user_pages* implementations")
moved get_user_pages_fast() from mm/util.c to mm/gup.c, but didn't update
the documentation, leading to this build warning:

  ./mm/util.c:1: warning: 'get_user_pages_fast' not found

Update the docs to match the new reality.

Fixes: 050a9adc6438 ("mm: consolidate the get_user_pages* implementations")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>