Add some JSDoc comments to rustdoc JS

This follows the Closure Compiler dialect of JSDoc, so we
can use it to do some basic type checking. We don't plan to
compile with Closure Compiler, just use it to check types. See
https://github.com/google/closure-compiler/wiki/ for details.

src/librustdoc/html/static/js/externs.js

@@ -0,0 +1,32 @@
+// This file contains type definitions that are processed by the Closure Compiler but are
+// not put into the JavaScript we include as part of the documentation. It is used for
+// type checking. See README.md in this directory for more info.
+
+/* eslint-disable */
+var searchState;
+function initSearch(searchIndex){}
+
+/**
+ * @typedef {{
+ *   raw: string,
+ *   query: string,
+ *   type: string,
+ *   id: string,
+ * }}
+ */
+var ParsedQuery;
+
+/**
+ * @typedef {{
+ *    crate: string,
+ *    desc: string,
+ *    id: number,
+ *    name: string,
+ *    normalizedName: string,
+ *    parent: (Object|null|undefined),
+ *    path: string,
+ *    ty: (Number|null|number),
+ *    type: (Array<?>|null)
+ * }}
+ */
+var Row;