Be clearer about how the bitrate fields in Flows should be set #236

Context
The Flow Profiles PR review pulled up some questions about whether max_bit_rate or avg_bit_rate make sense to appear in Profiles. The conclusion was to suggest that avg_bit_rate should, and should be seen as the target you might set an encoder to, whereas max_bit_rate behaves more like a measured value.

https://github.com/bbc/tams/blob/main/docs/appnotes/0013-setting-flow-bit-rate-properties.md suggests some approaches by which bit rate might be set, and how it might be used to calculate minimum buffer size etc.

It would be better if we were clearer about how these might be used, and the split between each. In general the bitrate of a piece of content is used for two purposes. While (2) can be used for (1) (and to a degree, vice versa) it is sometimes useful to indicate that quality even without any content, and be able to construct a Profile describing the approximate quality, so merging them may not be ideal.

1. Understanding broadly what quality to expect, when read alongside the codec and essence parameters
2. Understanding the minimum acceptable buffer size on a client to avoid a playback stall

Also tracked as https://jira.dev.bbc.co.uk/browse/CLOUDFIT-5601

Actions

• Write an ADR exploring options around how the two fields be used, e.g. using avg_bit_rate as more of a target than a measure
• Amend the appnote/fields in the spec to indicate that distinction

Outcome

• ADR on bitrates
• Spec/Appnote tweak