pgsql: Improve user-defined-aggregates documentation.

Improve user-defined-aggregates documentation.

On closer inspection, that two-element initcond value seems to have been
a little white lie to avoid explaining the full behavior of float8_accum.
But if people are going to expect the examples to be exactly correct,
I suppose we'd better explain. Per comment from Thom Brown.

Branch
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REL8_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/5b6c27a607641145d29b64c2f90907db0ed4b36f

Modified Files
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doc/src/sgml/xaggr.sgml | 13 +++++++++----
1 files changed, 9 insertions(+), 4 deletions(-)