pgsql: Improve user-facing documentation for partial/parallel aggregati

Improve user-facing documentation for partial/parallel aggregation.

Add a section to xaggr.sgml, as we have done in the past for other
extensions to the aggregation functionality.  Assorted wordsmithing
and other minor improvements.

David Rowley and Tom Lane

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/2d673424faf3e33c5fcca926fbe3f21e16dd0fef

Modified Files
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doc/src/sgml/ref/create_aggregate.sgml |  71 ++++++++++++++---------
doc/src/sgml/xaggr.sgml                | 100 ++++++++++++++++++++++++++++++++-
2 files changed, 142 insertions(+), 29 deletions(-)