ibaker b43db3bceb Clarify SSA and SubRip docs in MatroskaExtractor
The handling of times wasn't really clear to me, hopefully this more
exhaustive documentation helps a bit.

Also assert the end timecode is the 'correct' length for the format.

PiperOrigin-RevId: 279922369
2019-11-15 05:14:35 +00:00
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2019-11-15 05:14:27 +00:00