[doc] Add doc for QtAndroid::Runnable

Change-Id: Ib9d88dee71459d61d12bb975e24e8e59e687434d
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/androidextras/android/qandroidfunctions.cpp

@@ -180,6 +180,12 @@ void QtAndroid::startIntentSender(const QAndroidJniObject &intentSender,
 
 }
 
+/*!
+    \typedef QtAndroid::Runnable
+
+    Synonym for std::function<void()>.
+*/
+
 /*!
   \since 5.7
   \fn void QtAndroid::runOnAndroidThread(const Runnable &runnable)