Clarify that the `labels` config in dbt-bigquery is not additive

### Contributions

- [X] I have read the contribution docs, and understand what's expected of me.

### Link to the page on docs.getdbt.com requiring updates

https://docs.getdbt.com/reference/resource-configs/bigquery-configs#labels-and-tags

### What part(s) of the page would you like to see updated?

Explicitly mention that the `labels` configuration is [not additive -- it "clobbers"](https://docs.getdbt.com/reference/configs-and-properties#combining-configs) (overwrites) any previously configured value.

### Additional information

Instigated by https://github.com/dbt-labs/dbt-adapters/issues/587