TopMenu extension: specify which Module is needed

In the TopMenu developers' documentation it is needed
to indicate in which Plugin Module the extension needs
to be registered (if plugin is not auto-discoverable).

A common mistake is to reuse the Gerrit-HttpModule and thus
wondering why the menubar is not populated.

Change-Id: I949f5d178cb934c830082cd580f306107ecadd9a

Documentation/dev-plugins.txt

@@ -854,10 +854,13 @@ public class MyTopMenuExtension implements TopMenu {
 }
 ----
 
-Otherwise the top menu extension must be bound in a plugin module:
+Otherwise the top menu extension must be bound in the plugin module used
+for the Gerrit system injector (Gerrit-Module entry in MANIFEST.MF):
 
 [source,java]
 ----
+package com.googlesource.gerrit.plugins.helloworld;
+
 public class HelloWorldModule extends AbstractModule {
   @Override
   protected void configure() {
@@ -866,6 +869,12 @@ public class HelloWorldModule extends AbstractModule {
 }
 ----
 
+[source,manifest]
+----
+Gerrit-ApiType: plugin
+Gerrit-Module: com.googlesource.gerrit.plugins.helloworld.HelloWorldModule
+----
+
 [[http]]
 HTTP Servlets
 -------------