pgsql: Update autovacuum_freeze_max_age documentation to mention that

Log Message:
-----------
Update autovacuum_freeze_max_age documentation to mention that the
default is low because of pg_clog file removal.

Backpatch to 9.0.X.

Modified Files:
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pgsql/doc/src/sgml:
config.sgml (r1.307 -> r1.308)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.307&r2=1.308)