Describe the problem

Currently we can't retrieve builtin function comments via pg_objdescription, e.g. via

SELECT 
         p.proname AS "Name",
       pg_catalog.obj_description(p.oid, 'pg_proc') AS "Description"
     FROM pg_catalog.pg_proc p

This would be needed e.g. in:

• client-side \df handling via clisqlshell: new infrastructure for describe commands #88061
• auto-completion rule for functions in pkg/comprules/rules.go

Expected behavior

Built-in comments can be retrieved via pg_objdescription.

cc @rafiss for triage.
Epic: CRDB-23454

Jira issue: CRDB-23456