Clarify per-chunk columnstore settings, including segmentby keys

[atovpeko]

• Explain, where appropriate, that chunks do not need to be decompressed before changing the columnstore settings.
• Clarify in the introduction of columnstore configuration (both when CREATE/ALTER, but also when changing policies), that configurations are on a per-chunk basis, and what that means for evolution.

[atovpeko]

timescale/timescaledb#6513
timescale/timescaledb#6545
https://github.com/timescale/timescaledb/blob/main/CHANGELOG.md#2140-2024-02-08