[Docs RFC] Update API ref about time buckets and gapfilling

[billy-the-fish]

Use information from the thread in https://timescaledb.slack.com/archives/C4GT3N90X/p1727879988113139?thread_ts=1727313083.142569&cid=C4GT3N90X (edited) to update the docs in https://docs.timescale.com/use-timescale/latest/hyperfunctions/gapfilling-interpolation/ and https://docs.timescale.com/api/latest/hyperfunctions/gapfilling/time_bucket_gapfill/. Use information from https://www.timescale.com/blog/sql-functions-for-time-series-analysis/.