Improved FlxMouse documentation

In hindsight, the newly added functions in `FlxMouse` (#352) probably require a better documentation - namely that the `FLX_MOUSE_ADVANCED` flag needs to be set in order to be able to use them. Otherwise, this could cause confusion, as the functions will appear in auto-completion, but not really be available / cause compiler errors. To someone who hasn't used compiler conditionals yet, this could cause a lot of trouble - I guess it's not that much of an issue for the `FLX_NO`-Style conditionals, as they remove features on demand rather than adding them.