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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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/17f644911b6818837d2434032f53947e4a727740

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(-)