Add game state boolean ref docs (#1694)

* Add game state boolean doc refs

* Add check for _img validity

* JS guru tricks

docs/reference/game/is-game-over.md

@@ -0,0 +1,25 @@
+# is Game Over
+
+Find out if the game is over or not.
+
+```sig
+game.isGameOver()
+```
+
+## Returns
+
+* a [boolean](/types/boolean) value that is `true` if the game is over or `false` if not.
+# Example
+
+Be kind and give the player some points for trying.
+
+```blocks
+if (game.isGameOver() && game.score() < 10) {
+    game.addScore(10 - game.score())
+}
+```
+
+## See also
+
+[is running](/reference/game/is-running),
+[is paused](/reference/game/is-paused)