Document known limitations to `--empty` flag

### 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/commands/run#the---empty-flag
- https://docs.getdbt.com/reference/commands/build#the---empty-flag

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

The `--empty` flag does not work for:
- Already-aliased refs on adapters that don't support double-aliasing (i.e. Postgres)
- https://github.com/dbt-labs/dbt-adapters/issues/199
- Selecting pseudo-columns (i.e. on BigQuery)
- https://github.com/dbt-labs/dbt-bigquery/issues/1243
- ~Introspective queries, such as `get_columns_in_relation` on certain adapters, until we resolve those issues adapter-by-adapter~
- https://github.com/dbt-labs/dbt-adapters/issues/213
- https://github.com/dbt-labs/dbt-snowflake/issues/1033
- https://github.com/databricks/dbt-databricks/issues/807

### Additional information

_No response_