pgsql: Enable parallel query by default.

Enable parallel query by default.

Change max_parallel_degree default from 0 to 2.  It is possible that
this is not a good idea, or that we should go with 1 worker rather
than 2, but we won't find out without trying it.  Along the way,
reword the documentation for max_parallel_degree a little bit to
hopefully make it more clear.

Discussion: 20160420174631.3qjjhpwsvvx5bau5@alap3.anarazel.de

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/77cd477c4ba885cfa1ba67beaa82e06f2e182b85

Modified Files
--------------
doc/src/sgml/config.sgml                      | 12 ++++++++----
src/backend/optimizer/path/costsize.c         |  2 +-
src/backend/utils/misc/guc.c                  |  2 +-
src/backend/utils/misc/postgresql.conf.sample |  2 +-
4 files changed, 11 insertions(+), 7 deletions(-)