[Content Bug]Update chunk skipping docs to point out that it only works on chunks created after chunk skipping is enabled

[billy-the-fish]

Update https://docs.timescale.com/use-timescale/latest/hypertables/improve-query-performance/#when-to-enable-chunk-skipping and the API to say that it only applies to chunks created after chunk skipping is enabled, and add a note in the Hypercore docs saying about chunk skipping.

To be done after the hypercore docs are released.