Write documentation for input

src/input/button_state.rs

@@ -1,5 +1,9 @@
+/// The state of a button.
 #[derive(Debug, Hash, Ord, PartialOrd, PartialEq, Eq, Clone, Copy)]
 pub enum ButtonState {
+    /// The button is pressed.
     Pressed,
+
+    /// The button is __not__ pressed.
     Released,
 }