Thread: pgsql: Document that autovacuum_freeze_max_age is used for pg_clog

Log Message:
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Document that autovacuum_freeze_max_age is used for pg_clog recycling.
We already mentioned xid wraparound.

Modified Files:
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    pgsql/doc/src/sgml:
        config.sgml (r1.305 -> r1.306)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.305&r2=1.306)