docs: link bus and tasks

Add a link from bus section docs to the task docs. Add a paragraph to task docs
to tell about messages and the bus.

gst/gstbus.c

@@ -25,7 +25,8 @@
  * @see_also: #GstMessage, #GstElement
  *
  * The #GstBus is an object responsible for delivering #GstMessage packets in
- * a first-in first-out way from the streaming threads to the application.
+ * a first-in first-out way from the streaming threads (see #GstTask) to the
+ * application.
  *
  * Since the application typically only wants to deal with delivery of these
  * messages from one thread, the GstBus will marshall the messages between

gst/gsttask.c

@@ -56,6 +56,10 @@
  * After creating a #GstTask, use gst_object_unref() to free its resources. This can
  * only be done it the task is not running anymore.
  *
+ * Task functions can send a #GstMessage to send out-of-band data to the
+ * application. The application can receive messages from the #GstBus in its
+ * mainloop.
+ *
  * Last reviewed on 2006-02-13 (0.10.4)
  */