pgsql: Doc: Add references for SI and SSI.

Doc: Add references for SI and SSI.

Our documentation failed to point out that REPEATABLE READ is really
snapshot isolation, which might be important to some users.  Point to
the standard reference paper for this complicated topic.

Likewise, add a reference to the VLDB paper about PostgreSQL SSI, for
technical information about our SSI implementation and how it compares
to S2PL.

While here, add a note about catalog access using a lower isolation
level, per recent user complaint.

Back-patch to all releases.

Reported-by: Kyle Kingsbury <aphyr@jepsen.io>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Tatsuo Ishii <ishii@sraoss.co.jp>
Discussion: https://postgr.es/m/db7b729d-0226-d162-a126-8a8ab2dc4443%40jepsen.io
Discussion: https://postgr.es/m/16454-9408996bb1750faf%40postgresql.org

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e2aca0d02d97509d592669ec573c203ac1c4202d

Modified Files
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doc/src/sgml/biblio.sgml | 58 ++++++++++++++++++++++++++++++++++++++++++++++++
doc/src/sgml/mvcc.sgml   | 33 +++++++++++++++++++++++++++
2 files changed, 91 insertions(+)