sql: prevent arbitrary writes to system.comments

[jseldess]

PR: cockroachdb/cockroach#45712

From release notes:

All users can now view any comments on any object (bypassing other privileges), but modifying comments require write privilege on the target object. Previously, any user could modify any database/table/view/index comment via direct SQL updates to system.comments. This was unintended and a form of privilege escalation, and is now prevented. The privileges required for the COMMENT statement, pg_description, col_description(), obj_description(), and shobj_description() are operating as in PostgreSQL and are unaffected by this change.[#45712][#45712] {% comment %}doc{% endcomment %}

[ericharmeling]

We currently don't document system.comments, or really any system tables. We actually don't document privileges on any internal tables. I think we can close this issue, in favor of #6272.

Also related: #1291 and #2957