Clarify that Broker caching for groupBy v2 queries does not work

[FrankChen021]

Description

Broker caching has been disabled for groupBy v2 queries since #3950, and this limitation is stated under the section Differences between v1 and v2 of groupBy query doc. But since that's a large block of text, it's not intuitive for users to notice this constraint.

This PR adds a note under the section Broker Caching of the configuration doc to give users a more clear hint when they follow that doc to enable broker caching.

This PR has:

• been self-reviewed.

[sthetland]

Good idea to make this more prominent. Just one suggestion to put the groupBy term into its usual form.

[capistrant]

is groupBy v2 only not cacheable for the non-result cache or is it also not cacheable for result cache? If it is only for non-result cache, I think we should explicitly call that out to not confuse people.

[FrankChen021]

@capistrant Both of per-segment caching and whole-query caching are not supported for groupBy v2 queries. It's a good idea to put this in the doc to make it more clear.

[capistrant]

LGTM. the trick will be for someone to remember to remove this from the docs if down the road some day someone implements broker caching for groupby v2

[FrankChen021]

Hi @sthetland , Do you have any other comments ?