pgsql: Mention 'void' as the proper return type when there's nothing to

Log Message:
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Mention 'void' as the proper return type when there's nothing to
return, per suggestion from Joachim Wieland.

Modified Files:
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pgsql/doc/src/sgml/ref:
create_function.sgml (r1.68 -> r1.69)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_function.sgml.diff?r1=1.68&r2=1.69)