pipeline: clarify that applications should handle bus messages

The pipeline posts messages on the bus even if an application does not
handle the messages. This is expected behavior but may leak messages if
the messages are not handled.

Clarify the documentation.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/680>

gst/gstpipeline.c

@@ -40,8 +40,12 @@
  * methods like gst_bin_add() and gst_bin_remove() (see #GstBin).
  *
  * Before changing the state of the #GstPipeline (see #GstElement) a #GstBus
- * can be retrieved with gst_pipeline_get_bus(). This bus can then be
- * used to receive #GstMessage from the elements in the pipeline.
+ * should be retrieved with gst_pipeline_get_bus(). This #GstBus should then
+ * be used to receive #GstMessage from the elements in the pipeline. Listening
+ * to the #GstBus is necessary for retrieving error messages from the
+ * #GstPipeline and otherwise the #GstPipeline might stop without any
+ * indication, why. Furthermore, the #GstPipeline posts messages even if
+ * nobody listens on the #GstBus, which will pile up and use up memory.
  *
  * By default, a #GstPipeline will automatically flush the pending #GstBus
  * messages when going to the NULL state to ensure that no circular