Add comment re: quickhelp key.

IPython/frontend/html/notebook/static/js/notebook.js

@@ -261,6 +261,7 b' var IPython = (function (IPython) {'
                 that.control_key_active = false;
                 return false;
             } else if (event.which === 90 && that.control_key_active) {
+                // Undo last cell delete = z
                 that.undelete();
                 that.control_key_active = false;
                 return false;