pgsql: Doc: update documentation for requirement of ORDER BY inGROUPS

Doc: update documentation for requirement of ORDER BY in GROUPS mode.

Commit ff4f88916 adjusted the code to enforce the SQL spec's requirement
that a window using GROUPS mode must have an ORDER BY clause.  But I missed
that the documentation explicitly said you didn't have to have one.

Also minor wordsmithing in the window-function section of select.sgml.

Per Masahiko Sawada, though I didn't use his patch.

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/663060e8c3832ea8388e4ae128420f76680e5fe9

Modified Files
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doc/src/sgml/ref/select.sgml | 6 +++---
doc/src/sgml/syntax.sgml     | 4 ++--
2 files changed, 5 insertions(+), 5 deletions(-)