net: add documentation for io_uring zcrx

Add documentation for io_uring zero copy Rx that explains requirements
and the user API.

Signed-off-by: David Wei <dw@davidwei.uk>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20250215000947.789731-11-dw@davidwei.uk


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