err: mixing aggregate and non-aggregate queries is not supported

Question re error message.

__Proposal:__
Can we update msg to be more explicit?

__Current error:__
err: mixing aggregate and non-aggregate queries is not supported

__Proposed:__
SELECT clause cannot include an aggregate function and a non-aggregate function, field key, or tag key.

(if this statement covers all use cases where folks would get this error?)

__Use case:__
User interpreted this error occurred because sets of fields in SELECT and GROUP BY were overlapping. Related [doc issue](https://github.com/influxdata/docs.influxdata.com/issues/2654).

Start by locating the code that emits “err: mixing aggregate and non-aggregate queries is not supported”; the payload names no file or test. Compare its behavior with the proposed SELECT-clause wording and the related documentation issue, then verify that the revised message covers the reported use case without implying that overlapping SELECT and GROUP BY fields cause the error.

Written by the indexing model from the issue text.