Rewrite window function reference section to more clearly explain

Bruce Momjian <bruce@momjian.us>

Commit: d5816d11af6334e94132bc73939d7c12b3cff792
Author: Bruce Momjian <bruce@momjian.us>
Date: 2012-08-16T14:42:42Z
Releases: 9.3.0
Rewrite window function reference section to more clearly explain
keywords and concepts, based on suggestions by Florian Pflug.

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doc/src/sgml/syntax.sgml modified +41 −27