Clarify legacy logs format trace context flags and tracestate

[jmacd]

Part of #3303.

Changes

Clarifies how the trace flags are represented when placing trace context in log records. Clarifies that tracestate is not encoded when placing trace context in log records.

• [x] Related issues #
• [ ] Related OTEP(s) #
• [ ] Links to the prototypes (when adding or changing features)
• [x] CHANGELOG.md file updated for non-trivial changes
• [x] spec-compliance-matrix.md updated if necessary