liburing: add man pages for helper functions.

This adds man pages for the following functions:
- io_uring_cq_advance
- io_uring_cq_ready
- io_uring_free_probe
- io_uring_get_probe
- io_uring_opcode_supported
- io_uring_sq_ready
- io_uring_sq_space_left
- io_uring_sqe_set_flags
- io_uring_sqring_wait

Signed-off-by: Stefan Roesch <shr@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
9 files changed