More explicit guidance on custom partitioning (refinement round)

[reidkaufmann]

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.