pgsql: Doc: clarify behavior of standard aggregates for null inputs.

Doc: clarify behavior of standard aggregates for null inputs.

Section 4.2.7 says that unless otherwise specified, built-in
aggregates ignore rows in which any input is null. This is
not true of the JSON aggregates, but it wasn't documented.
Fix that.

Of the other entries in table 9.55, some were explicit about
ignoring nulls, and some weren't; for consistency and
self-contained-ness, make them all say it explicitly.

Per bug #15884 from Tim Möhlmann. Back-patch to all supported
branches.

Discussion: https://postgr.es/m/15884-c32d848f787fcae3@postgresql.org

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5efa4a3e7008638732d14ffd7613f53823a75367

Modified Files
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doc/src/sgml/func.sgml | 26 +++++++++++++++-----------
1 file changed, 15 insertions(+), 11 deletions(-)