Update custom.js

Explain different semantics required for instances versus functions or classes. Based on comments of @minrk in  #5985.

IPython/html/static/custom/custom.js

@@ -1,58 +1,68 b''
 // leave at least 2 line with only a star on it below, or doc generation fails
 /**
  *
  *
  * Placeholder for custom user javascript
  * mainly to be overridden in profile/static/custom/custom.js
  * This will always be an empty file in IPython
  *
  * User could add any javascript in the `profile/static/custom/custom.js` file
  * (and should create it if it does not exist).
  * It will be executed by the ipython notebook at load time.
  *
  * Same thing with `profile/static/custom/custom.css` to inject custom css into the notebook.
+ * 
+ * Classes and functions are available at load time and may be accessed plainly: 
+ * 
+ *     IPython.Cell.options_default.cm_config.extraKeys = {Home: "goLineLeft", End: "goLineRight"};
+ * 
+ * Instances are created later however and must be accessed using events:
+ * 
+ *     $([IPython.events]).on("app_initialized.NotebookApp", function () {
+ *         IPython.keyboard_manager....
+ *     });
  *
  * __Example 1:__
  *
  * Create a custom button in toolbar that execute `%qtconsole` in kernel
  * and hence open a qtconsole attached to the same kernel as the current notebook
  *
  *    IPython.events.on('app_initialized.NotebookApp', function(){
  *        IPython.toolbar.add_buttons_group([
  *            {
  *                 'label'   : 'run qtconsole',
  *                 'icon'    : 'icon-terminal', // select your icon from http://fortawesome.github.io/Font-Awesome/icons
  *                 'callback': function () {
  *                     IPython.notebook.kernel.execute('%qtconsole')
  *                 }
  *            }
  *            // add more button here if needed.
  *            ]);
  *    });
  *
  * __Example 2:__
  *
  * At the completion of the dashboard loading, load an unofficial javascript extension
  * that is installed in profile/static/custom/ 
  *
  *    IPython.events.on('app_initialized.DashboardApp', function(){
  *        require(['custom/unofficial_extension.js'])
  *    });
  *
  * __Example 3:__
  *
  *  Use `jQuery.getScript(url [, success(script, textStatus, jqXHR)] );`
  *  to load custom script into the notebook.
  *
  *    // to load the metadata ui extension example.
  *    $.getScript('/static/notebook/js/celltoolbarpresets/example.js');
  *    // or
  *    // to load the metadata ui extension to control slideshow mode / reveal js for nbconvert
  *    $.getScript('/static/notebook/js/celltoolbarpresets/slideshow.js');
  *
  *
  * @module IPython
  * @namespace IPython
  * @class customjs
  * @static
  */