Minor Description
As spotted on StackOverflow <https://stackoverflow.com/q/64888488/685641> and later discussed on the mailing list <https://lists.apache.org/thread.html/r3b9fd864a9ab897bf0f2a53c2d6fd5f770b9688f8425ab0877e20b6b%40%3Cdev.tika.apache.org%3E>, the units of `xmpDM:duration` should be seconds by default.
Most of our parsers already assume that, but the MP3 one doesn't.
So, need to fix it to also output seconds not milliseconds, as well as documenting the the duration will be in seconds unless there's also a scale set