Description
Is your feature request related to a problem? Please describe.
In #96562, we're building a heuristic by which some completion results (pg_catalog table names) are hidden by default. However, the user won't know this from the get go and it is relatively hard to discover.
So we'd like a mechanism through which completion rules can show more information on the screen than just completions, for example to suggest alternate ways to reach information that's hidden.
Describe the solution you'd like
The completion framework should be able to pump additional informational texts through the output of SHOW COMPLETIONS, in a way that the SQL shell would then interpret and show on the screen.
This might be achievable through SQL NOTICES (we'd need to check these are properly displayed on a completion context), or perhaps some new/different result row types in the SHOW COMPLETIONS tabular output.
Jira issue: CRDB-24274