Document that op_bitrate() is for all streams.
If an Opus stream is currently multiplexed with other streams, we would have to scan the whole file to identify the pages that belong to just the stream we play back. That's probably not what the user wants, and if it is, they have a way to do that (using op_bitrate_instant()). Document this so as not to surprise developers. Thanks to Lithopsian for the report.