pgsql: Copy-editing for recent window-functions documentation rewrite.

Copy-editing for recent window-functions documentation rewrite.

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

Branch
------
master

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

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