Add comment about applicationCache not supporting CORS

assets/javascripts/app/app.coffee

@@ -97,6 +97,8 @@
     return
 
   indexHost: ->
+    # Can't load the index files from the host/CDN when applicationCache is
+    # enabled because it doesn't support caching URLs that use CORS.
     @config[if @appCache and @settings.hasDocs() then 'index_path' else 'docs_host']
 
   onBootError: (args...) ->