io_uring: improve req fields comments

Move a misplaced comment about req->creds and add a line with
assumptions about req->link.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/1e51d1e6b1f3708c2d4127b4e371f9daa4c5f859.1648209006.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>