pgsql: Doc: improve discussion of plpgsql's GET DIAGNOSTICS, other mino

Doc: improve discussion of plpgsql's GET DIAGNOSTICS, other minor fixes.

9.4 added a second description of GET DIAGNOSTICS that was totally
independent of the existing one, resulting in each description lying to the
extent that it claimed the set of status items it described was complete.
Fix that, and do some minor markup improvement.

Also some other small fixes per bug #14258 from Dilian Palauzov.

Discussion: <20160718181437(dot)1414(dot)40802(at)wrigleys(dot)postgresql(dot)org>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ade64d05a0c9c77def776f64ea399697c3cd7f61

Modified Files
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doc/src/sgml/plpgsql.sgml | 139 +++++++++++++++++++++++++-----------------
doc/src/sgml/release-9.4.sgml | 2 +-
2 files changed, 85 insertions(+), 56 deletions(-)