pgsql: Add a specific example of parenthesizing when extracting a field

Log Message:
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Add a specific example of parenthesizing when extracting a field of a
composite column. Might help cut down on future questions...

Modified Files:
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pgsql/doc/src/sgml:
syntax.sgml (r1.125 -> r1.126)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/syntax.sgml?r1=1.125&r2=1.126)