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

@@ -12,6 +12,16 b''
  *
  * 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