No minimum retention period, `0<h|d>` means data expires immediately

We should call this out explicitly. In 3.2, `retention period = 0` is supported and seems to mean "delete immediately". Historically, `0d = infinite` (see https://docs.influxdata.com/influxdb3/clustered/admin/databases/create/#retention-period-syntax)

[Slack Message](https://influxdata.slack.com/archives/C084G9LR2HL/p1751037184920229?thread_ts=1751037184.920229&cid=C084G9LR2HL)

Start with the linked InfluxDB 3.2 retention-period syntax documentation and compare its treatment of `0h|d|unit` with the historical `0d` behavior described in the issue. Update the relevant documentation to state explicitly what zero retention means, then verify the wording and examples are consistent.

Written by the indexing model from the issue text.