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
1 file changed