More explicit guidance on custom partitioning (refinement round)

We still have some ongoing confusion with a customer on the timeformat. We recently made good progress removing misleading examples, but we can be more clear and prominent stating there are _ONLY 3_ supported format strings:

- `%Y-%m-%d` for daily
- `%Y-%m` for monthly
- `%Y` for yearly

Positioned right in here:
https://docs.influxdata.com/influxdb/clustered/admin/custom-partitions/define-custom-partitions/

Also suggested by Support:

> I do think the documentation should have more of a call out on what time formats are supported at top. (Example time parts are only listed at bottom in the examples section). Also a flag called template-timeformat is confusing when only date parts are allowed:

> Potentially a new section `Partitioning by distinct date values`:

Keep in mind, the customer may not have read the most recent iteration of the documentation, which is quite good. My only concern is that we're clear that _no other time formats should be used_.