Point out in default_tablespace's description that CREATE DATABASE ignores it.

Per gripe from Andreas Scherbaum.

doc/src/sgml/config.sgml

@@ -4356,6 +4356,12 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         <xref linkend="guc-temp-tablespaces"> is consulted instead.
        </para>
 
+       <para>
+        This variable is also not used when creating databases.
+        By default, a new database inherits its tablespace setting from
+        the template database it is copied from.
+       </para>
+
        <para>
         For more information on tablespaces,
         see <xref linkend="manage-ag-tablespaces">.