pgsql: Make it clearer that not every Postgres character set can be used

Log Message:
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Make it clearer that not every Postgres character set can be used as a
server-side character set.

Tags:
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REL8_1_STABLE

Modified Files:
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pgsql/doc/src/sgml:
charset.sgml (r2.75 -> r2.75.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/charset.sgml.diff?r1=2.75&r2=2.75.2.1)