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: <gstreamer/gstreamer!680>
36 jobs for !680 with clarify-pipeline-bus-documentation in 29 minutes and 50 seconds (queued for 3 seconds)
merge request