Document main function in webapp

docs/source/packages/webapp.md

@@ -160,3 +160,4 @@ WebApp.addUpdatedNotifyHook(({arch, manifest, runtimeConfig}) => {
 
 {% apibox "WebApp.addUpdatedNotifyHook" %}
 {% apibox "addUpdatedNotifyHookCallback(options)" %}
+{% apibox "main" %}

packages/webapp/webapp_server.js

@@ -1360,6 +1360,13 @@ function runWebAppServer() {
     },
   });
 
+    /**
+   * @name main
+   * @locus Server
+   * @summary Starts the HTTP server.
+   *  If `UNIX_SOCKET_PATH` is present Meteor's HTTP server will use that socket file for inter-process communication, instead of TCP.
+   * If you choose to not include webapp package in your application this method still must be defined for your Meteor application to work. 
+   */
   // Let the rest of the packages (and Meteor.startup hooks) insert connect
   // middlewares and update __meteor_runtime_config__, then keep going to set up
   // actually serving HTML.