io_uring: add comments on how the async buffered read retry works

The retry based logic here isn't easy to follow unless you're already
familiar with how io_uring does task_work based retries. Add some
comments explaining the flow a little better.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>