Clarify docs for the thread used in BroadcastReceiver.onReceive()

    /**

     * This method is called when the BroadcastReceiver is receiving an Intent

     * broadcast.  During this time you can use the other methods on

     * BroadcastReceiver to view/modify the current result values.  The function

     * is normally called within the main thread of its process, so you should

     * BroadcastReceiver to view/modify the current result values.  This method

     * is always called within the main thread of its process, unless you

     * explicitly asked for it to be scheduled on a different thread using

     * {@link android.content.Context#registerReceiver(BroadcastReceiver,

     * IntentFilter, String, android.os.Handler)}. When it runs on the main

     * thread you should

     * never perform long-running operations in it (there is a timeout of

     * 10 seconds that the system allows before considering the receiver to

     * be blocked and a candidate to be killed). You cannot launch a popup dialog