Update autovacuum_freeze_max_age documentation to mention that the

default is low because of pg_clog file removal.

Backpatch to 9.0.X.

doc/src/sgml/config.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.307 2010/08/23 02:43:25 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.308 2010/08/24 13:32:25 momjian Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -4150,12 +4150,16 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         Specifies the maximum age (in transactions) that a table's
         <structname>pg_class</>.<structfield>relfrozenxid</> field can
-        attain before a <command>VACUUM</> operation is forced to allow removal
-        of old files from the <filename>pg_clog</> subdirectory and prevent
-        transaction ID wraparound within the table.  Note that the system
-        will launch autovacuum processes to prevent wraparound even when
-        autovacuum is otherwise disabled.
-        The default is 200 million transactions.
+        attain before a <command>VACUUM</> operation is forced
+        to prevent transaction ID wraparound within the table.
+        Note that the system will launch autovacuum processes to
+        prevent wraparound even when autovacuum is otherwise disabled.
+       </para>
+
+       <para>
+        Vacuum also allows removal of old files from the
+        <filename>pg_clog</> subdirectory, which is why the default
+        is a relatively low 200 million transactions.
         This parameter can only be set at server start, but the setting
         can be reduced for individual tables by
         changing storage parameters.