Copy-editing for recent window-functions documentation rewrite.

Fix grammar, put back some removed information, rearrange for clarity.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/18226849ea12c566fb2b3be505448e0ba289ea10

Modified Files
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doc/src/sgml/syntax.sgml |   68 ++++++++++++++++++++++++++++++---------------
1 files changed, 45 insertions(+), 23 deletions(-)

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Next:	From: Gavin Flower	Date: 2012-08-18 08:37:16
	Subject: Re: Re: [COMMITTERS] pgsql: In docs, change a few cases of "not important" to "unimportant".
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	Subject: pgsql: Check LIBXML_VERSION instead of testing in configure script.