Document that autovacuum_freeze_max_age is used for pg_clog recycling.

We already mentioned xid wraparound.

doc/src/sgml/config.sgml

@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.305 2010/08/19 22:55:00 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.306 2010/08/22 02:37:32 momjian Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -4150,7 +4150,8 @@ 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 prevent
+        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.