doc: Add a note that pipeline's bus queue is infinite

Xavier Claessens requested to merge xclaesse/gstreamer:doc-bus-leak into main

When using gst_pipeline_new() or gst_parse_launch() APIs, application MUST pop messages from the bus otherwise they accumulate infinitely and consume memory. This is unexpected behaviour that can only be spotted after hours/days of a running pipeline. Since it wouldn't be backward compatible to change enable-async property to FALSE by default, it should at least be properly documented.

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