Document known limitations to --empty flag

[jtcohen6]

Contributions

• 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)
  • [Feature] Support contextual table aliasing  dbt-adapters#199
• Selecting pseudo-columns (i.e. on BigQuery)
  • [Bug] --empty flag not working on Pseudo-columns dbt-bigquery#1243
• Introspective queries, such as get_columns_in_relation on certain adapters, until we resolve those issues adapter-by-adapter
  • [Bug] --empty flag generates invalid SQL when running adapter.get_columns_in_relation dbt-adapters#213
  • [Bug] --empty doesn't work when using get_columns_in_relation dbt-snowflake#1033

Additional information

No response