doc: Granting TRIGGER or REFERENCES on table is dangerous.

doc: Granting TRIGGER or REFERENCES on table is dangerous.

It's always been the case that granting these privileges to users that
you don't fully trust was a bad idea, but it hasn't always been
obvious to people reading the documentation that this is the case.
To prevent confusion, and also repeated reports to pgsql-security,
mention it explicitly.

Discussion: http://postgr.es/m/CA+TgmobrjCHBuWHrvX3=2vndUCO2thUOdevrCcMDFW86cqCYvw@mail.gmail.com
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Backpatch-through: 14