pgsql: Docs: provide a concrete discussion and example for RLS race con

Docs: provide a concrete discussion and example for RLS race conditions.

Commit 43cd468cf01007f3 added some wording to create_policy.sgml purporting
to warn users against a race condition of the sort that had been noted some
time ago by Peter Geoghegan. However, that warning was far too vague to be
useful (or at least, I completely failed to grasp what it was on about).
Since the problem case occurs with a security design pattern that lots of
people are likely to try to use, we need to be as clear as possible about
it. Provide a concrete example in the main-line docs in place of the
original warning.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/d878b115c350c18c0c9836c7652d5a179b5f9a33

Modified Files
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doc/src/sgml/ddl.sgml | 114 +++++++++++++++++++++++++++++++++++
doc/src/sgml/ref/create_policy.sgml | 12 +---
2 files changed, 116 insertions(+), 10 deletions(-)