Document null. Fixes #1

Author: tmcw

globals-docs.json

@@ -28,6 +28,7 @@
     "Math": "https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math",
     "NaN": "https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN",
     "Number": "https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number",
+    "null": "https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/null",
     "Object": "https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object",
     "parseFloat": "https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseFloat",
     "parseInt": "https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt",