pgsql: Document that chr(0) is not supported, and why.

Log Message:
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Document that chr(0) is not supported, and why.

Tags:
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REL8_3_STABLE

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.419 -> r1.419.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.419&r2=1.419.2.1)