btrfs: update function comment for btrfs_check_nocow_lock()

The documentation for the @nowait parameter is missing, so add it.
The @nowait parameter was added in commit 80f9d241 ("btrfs: make
btrfs_check_nocow_lock nowait compatible"), which forgot to update the
function comment.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>