document a few more keyboard primitives

reference.md

@@ -251,6 +251,15 @@ visible on screen.
 
 ### keyboard primitives
 
+* `App.is_cursor_movement(key)` -- return `true` if `key` is a cursor movement
+  key (arrow keys, page-up/down, home/end)
+
+* `App.cmd_down()`, `App.ctrl_down`, `App.alt_down()`, `App.shift_down()` --
+  predicates for different modifier keys.
+
+* `App.any_modifier_down()` -- returns `true` if any of the modifier keys is
+  currently pressed.
+
 * `App.key_down(key)` -- returns `true` if the given key is currently pressed.
   (Based on [LÖVE](https://love2d.org/wiki/love.keyboard.isDown).)