Document differences in type-handling for Flux and InfluxQL in Cloud

Related to https://github.com/influxdata/EAR/issues/3091

The InfluxDB Cloud storage engine allows users to write points with different types for the same field across different series (this is unique to Cloud—OSS will throw a type conflict error if you try to do this). We shouldn't encourage this practice; in fact we should discourage it, but it is technically possible on buckets without explicit schemas.

The issue comes when querying the data:

- Flux can handle the same field with different types across series and will return results successfully.
- InfluxQL cannot handle the same field with different types across series and will return an empty result.

##### Relevant URLs
- Flux vs InfluxQL - https://docs.influxdata.com/influxdb/cloud/reference/syntax/flux/flux-vs-influxql/
- Query data with InfluxQL - https://docs.influxdata.com/influxdb/cloud/query-data/influxql/
- InfluxDB schema design - https://docs.influxdata.com/influxdb/cloud/write-data/best-practices/schema-design/

Start with the linked Flux vs InfluxQL, Query data with InfluxQL, and InfluxDB schema design pages. Compare how each describes mixed field types across series and document that Flux returns results while InfluxQL returns an empty result, while discouraging this schema pattern. Verify that the guidance is consistent across the relevant Cloud documentation.

Written by the indexing model from the issue text.