ublk: document zero copy feature

Add words to explain how zero copy feature works, and why it has to be
trusted for handling IO read command.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250327095123.179113-8-ming.lei@redhat.com


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