Sentences about placement for show_frame = 0

[cconcolato]

The sentence:

HDR10+ Metadata OBUs are not provided when show_frame = 0.

is a consequence of the sentences in the previous paragraph, because:

• a show_frame = 0 cannot be on its own in a Temporal unit
• if the show_frame = 0 had a HDR10+ Metadata OBU, either the TU would have 2 HDR10+ Metadata OBU and that is invalid, or the show_frame = 1 in the same TU would not have an HDR10+ Metadata OBU, and this is also invalid.

We should:

• remove the assert so that it does not appear in the conformance report
• consider making the sentence a note to make it clear that it is not an additional statement

The second sentence:

For non-layered streams, there is only one HDR10+ Metadata OBU per temporal unit.

should also not be wrapped into an assert and maybe put into a NOTE. It's a rephrase/consequence of the previous paragraph.

[cconcolato]

@podborski I think this issue can be closed, correct?