man/io_uring_register.2: note when MAX_UNBOUND became available

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/man/io_uring_register.2 b/man/io_uring_register.2
index d38d668..e9a5ca4 100644
--- a/man/io_uring_register.2
+++ b/man/io_uring_register.2
@@ -338,6 +338,8 @@
 .I nr_args
 must be set to 1, as the command only takes one argument.
 
+Available since 5.15.
+
 .SH RETURN VALUE
 
 On success,