analytics: Add comment explaining server admin routes.

analytics/urls.py

@@ -4,6 +4,7 @@ from zerver.lib.rest import rest_dispatch
 import analytics.views
 
 i18n_urlpatterns = [
+    # Server admin (user_profile.is_staff) visible stats pages
     url(r'^activity$', analytics.views.get_activity,
         name='analytics.views.get_activity'),
     url(r'^realm_activity/(?P<realm_str>[\S]+)/$', analytics.views.get_realm_activity,