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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master

Details
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http://git.postgresql.org/pg/commitdiff/60270e5e00850ee8cc34296e38d0000415c8b152

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(+)