Clarify what a sample is for FlowSegment sample_offset/count

[philipnbbc]

Details

This PR adds some description text to clarify what is meant by a sample is in the definition of the FlowSegment sample_offset and sample_count properties.

Note that there is a general discussion about whether FlowSegment should reference the coded media units or presentation media units. The result of that discussion may be that the definition and naming of these properties changes.

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