Doc: add a summary table to the CREATE POLICY docs.

This table summarizes which RLS policy expressions apply to each
command type, and whether they apply to the old or new tuples (or
both), which saves reading through a lot of text.

Rod Taylor, hacked on by me. Reviewed by Fabien Coelho.

Discussion: https://postgr.es/m/CAHz80e4HxJShm6m9ZWFrHW=pgd2KP=RZmfFnEccujtPMiAOW5Q@mail.gmail.com