opt: add some comment

config.conf

@@ -152,6 +152,8 @@ bind=CTRL,Right,viewtoright_have_client,
 bind=CTRL+SUPER,Left,tagtoleft,
 bind=CTRL+SUPER,Right,tagtoright,
 
+# normal num key  is (1-9)
+# right-side keyboard num keys is (KP_1-KP_9)
 bind=Ctrl,KP_1,view,1
 bind=Ctrl,KP_2,view,2
 bind=Ctrl,KP_3,view,3