Add clarifying comment on fileenginehandler registration

The instantiated file engine handlers inherit from the private
QAbstractFileEngineHandler class, instantiation means also registration
(~RAII) for Qt file engine classes. This may not be obvious, so
perhaps a clarifying comment is welcome.

Change-Id: Ie8793060a62559e7550d4f808dec947fd5f09ca7
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>

src/plugins/platforms/android/androidjnimain.cpp

@@ -364,6 +364,7 @@ static jboolean startQtAndroidPlugin(JNIEnv *env, jobject /*object*/, jstring pa
         return false;
 
     m_androidPlatformIntegration = nullptr;
+    // File engine handler instantiation registers the handler
     m_androidAssetsFileEngineHandler = new AndroidAssetsFileEngineHandler();
     m_androidContentFileEngineHandler = new AndroidContentFileEngineHandler();
     m_androidApkFileEngineHandler = new QAndroidApkFileEngineHandler();