pgsql: Add a note about preferred window width to the section about code

Log Message:
-----------
Add a note about preferred window width to the section about code
formatting conventions.

Modified Files:
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pgsql/doc/src/sgml:
sources.sgml (r2.36 -> r2.37)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/sources.sgml?r1=2.36&r2=2.37)