pgsql: Add more parallel query documentation.

Add more parallel query documentation.

Previously, the individual settings were documented, but there was
no overall discussion of the capabilities and limitations of the
feature.  Add that.

Patch by me, reviewed by Peter Eisentraut and Álvaro Herrera.

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/c925e6335e1da58aba3dd764bb9f97fa81d893b8

Modified Files
--------------
doc/src/sgml/config.sgml   |   5 +
doc/src/sgml/filelist.sgml |   1 +
doc/src/sgml/parallel.sgml | 472 +++++++++++++++++++++++++++++++++++++++++++++
doc/src/sgml/postgres.sgml |   1 +
4 files changed, 479 insertions(+)