Only variables defined with `--vars` are available within the `dbt_project.yml` context

### 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

These pages should all be considered for updates:
- https://docs.getdbt.com/reference/project-configs/query-comment
- https://docs.getdbt.com/docs/build/project-variables
- https://docs.getdbt.com/reference/dbt-jinja-functions/var

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

As described in https://github.com/dbt-labs/dbt-core/issues/8869, make it clear that only variables defined with `--vars` are available when using [`var()`](https://docs.getdbt.com/reference/dbt-jinja-functions/var) within the [`dbt_project.yml` context](https://docs.getdbt.com/reference/dbt-jinja-functions/dbt-project-yml-context).

### Additional information

_No response_