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/main.js

@@ -420,6 +420,13 @@ function hideThemeButtonState() {
         return document.getElementById("help");
     }
 
+    /**
+     * Show the help popup.
+     *
+     * @param {boolean} display    - Whether to show or hide the popup
+     * @param {Event}   ev         - The event that triggered this call
+     * @param {Element} [help]     - The help element if it already exists
+     */
     function displayHelp(display, ev, help) {
         if (display) {
             help = help ? help : getHelpElement(true);