Doc: Further fix CREATE FOREIGN TABLE synopsis.

This patch fixes the partitioning synopsis in the Parameters section in
the CREATE FOREIGN TABLE documentation.  Follow-up for commit ce21a36cf.

Back-patch to v11 where default partition was introduced.

Reviewed by Amit Langote and Robert Haas.

Discussion: https://postgr.es/m/CAPmGK17U5jEqVZuo3r38wB0VFWomEtJCBGn_h92HQzQ2sP-49Q%40mail.gmail.com

doc/src/sgml/ref/create_foreign_table.sgml

@@ -173,7 +173,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
    </varlistentry>
 
    <varlistentry>
-    <term><literal>PARTITION OF <replaceable>parent_table</replaceable> FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable></literal></term>
+    <term><literal>PARTITION OF <replaceable>parent_table</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT }</literal></term>
     <listitem>
      <para>
       This form can be used to create the foreign table as partition of