Doc: fix oversimplified example for CREATE POLICY.

As written, this policy constrained only the post-image not the pre-image
of rows, meaning that users could delete other users' rows or take
ownership of such rows, contrary to what the docs claimed would happen.
We need two separate policies to achieve the documented effect.

While at it, try to explain what's happening a bit more fully.

Per report from Олег Самойлов.  Back-patch to 9.5 where this was added.
Thanks to Stephen Frost for off-list discussion.

Discussion: https://postgr.es/m/3298321532002010@sas1-2b3c3045b736.qloud-c.yandex.net