man: document IORING_TIMEOUT_IMMEDIATE_ARG flag

Document the new IORING_TIMEOUT_IMMEDIATE_ARG flag which allows storing
the timeout value directly in the SQE as nanoseconds rather than as a
pointer to a struct __kernel_timespec.

Fixes: ac9b8d0daedc ("tests: test timeout with immediate arguments")
Signed-off-by: Jens Axboe <axboe@kernel.dk>