pgsql: In docs, clarify that, without ORDER BY, the window frame is all

In docs, clarify that, without ORDER BY, the window frame is all rows in
the partition.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/c6ce4c13229c64c01393c86be220b532a0244d87

Modified Files
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doc/src/sgml/syntax.sgml |    7 ++++---
1 files changed, 4 insertions(+), 3 deletions(-)