[COMMITTERS] pgsql: Doc: add example of transition table use in a trigger.

Doc: add example of transition table use in a trigger. I noticed that there were exactly no complete examples of use of a transition table in a trigger function, and no clear description of just how you'd do it either. Improve that. Branch -- REL_10_STABLE Details --- https://git.postgr

[COMMITTERS] pgsql: Doc: add example of transition table use in a trigger.

Doc: add example of transition table use in a trigger. I noticed that there were exactly no complete examples of use of a transition table in a trigger function, and no clear description of just how you'd do it either. Improve that. Branch -- master Details --- https://git.postgresql.or