Add comment descripting externs

Bug: Issue 7022
Change-Id: Icef8047c9faf3bd3912040a7ffe5b6fdc9a6c873

polygerrit-ui/app/template_test_srcs/template_test.js

@@ -1,6 +1,14 @@
 const fs = require('fs');
 const twinkie = require('fried-twinkie');
 
+/**
+ * For the purposes of template type checking, externs should be added for
+ * anything set on the window object. Note that sub-properties of these
+ * declared properties are considered something separate.
+ *
+ * @todo (beckysiegel) Gerrit's class definitions should be recognized in
+ *    closure types.
+ */
 const EXTERN_NAMES = [
   'Gerrit',
   'GrAnnotation',