pgsql: Doc: fix oversimplified example for CREATE POLICY.

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

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/506bbbf02e8c045b6d35dc3b0083e5a12f7b3837

Modified Files
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doc/src/sgml/ddl.sgml | 36 ++++++++++++++++++++++++++++++------
1 file changed, 30 insertions(+), 6 deletions(-)