expand refint docs with usage info

The security team has received a couple of reports about potential SQL
injection opportunities via refint's trigger arguments. We discussed this
while preparing CVE-2026-6637 and concluded that forcibly quoting these
arguments would be much more likely to break working code than to prevent
any exploits. Unlike data values, the table/column names come from trigger
arguments, and there is little reason for a trigger author to put hostile
inputs into those arguments.

The attached documentation patch was originally intended to go along with
CVE-2026-6637, but we ultimately scoped it down to only the
security-relevant parts. This should be back-patched to v14. Note that we
are preparing to removing refint completely in v20, but IMHO this doc
update is still worth doing.

Thoughts?

--
nathan