Doc: Fix QDoc warnings in ExtensionSystem class docs

Task-number: QTCREATORBUG-23544
Change-Id: I6500fefdaa919f807fa4bf4372426b19a6e57aa2
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/extensionsystem/pluginmanager.cpp

@@ -71,6 +71,7 @@ enum { debugLeaks = 0 };
 
 /*!
     \namespace ExtensionSystem
+    \inmodule QtCreator
     \brief The ExtensionSystem namespace provides classes that belong to the
            core plugin system.
 
@@ -85,6 +86,7 @@ enum { debugLeaks = 0 };
 
 /*!
     \class ExtensionSystem::PluginManager
+    \inmodule QtCreator
     \ingroup mainclasses
 
     \brief The PluginManager class implements the core plugin system that