man: note when IORING_REGISTER_SEND_MSG_RING became available

The io_uring_register.2 man page had it listed as 6.12, which is
incorrect. And the io_uring_register_sync_msg.3 didnt have a note at
all.

Note that support for this feature arrived with kernel 6.13.

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

Author: axboe

man/io_uring_register.2

@@ -923,7 +923,7 @@ which must be prepared with
 .BR io_uring_prep_msg_ring (3)
 before being submitted. Only supports
 .B IORING_MSG_DATA
-type of requests. Available since kernel 6.12.
+type of requests. Available since kernel 6.13.
 
 .TP
 .B IORING_REGISTER_RESIZE_RINGS

man/io_uring_register_sync_msg.3

@@ -63,6 +63,8 @@ field on CQE side. That way, you can set the CQE flags field
 when sending a message. Be aware that io_uring could potentially set additional
 bits into this field.
 
+Available since kernel 6.13.
+
 .SH RETURN VALUE
 Returns 0 on success, or
 .BR -errno