Improve comments about startup notification

Do a better job at describe the problems in docDisplay.js related
to not reading the desktop files and finding StartupNotify=true.
Also, fix a typo.

http://bugzilla.gnome.org/show_bug.cgi?id=580658

js/ui/main.js

@@ -176,7 +176,7 @@ function create_app_launch_context() {
     context.set_timestamp(display.get_current_time());
 
     // Make sure that the app is opened on the current workspace even if
-    // the user switches before it seetarts
+    // the user switches before it starts
     context.set_desktop(screen.get_active_workspace_index());
 
     return context;