pgsql: Add an example of WITH (UPDATE RETURNING) INSERT to the INSERT r

Add an example of WITH (UPDATE RETURNING) INSERT to the INSERT ref page.

Per a discussion with Gavin Flower. This barely scratches the surface
of potential WITH (something RETURNING) use cases, of course, but it's
one of the simplest compelling examples I can think of.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8ad58279fa75636b776e4c648e1ffc7079a2c7d9

Modified Files
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doc/src/sgml/ref/insert.sgml | 14 ++++++++++++++
1 files changed, 14 insertions(+), 0 deletions(-)