Add markup for new constraint_name mention.

doc/src/sgml/ref/create_table.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/create_table.sgml,v 1.97 2006/01/16 19:50:03 momjian Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/create_table.sgml,v 1.98 2006/01/16 19:53:12 momjian Exp $
 PostgreSQL documentation
 -->
 
@@ -295,9 +295,10 @@ and <replaceable class="PARAMETER">table_constraint</replaceable> is:
      <para>
       An optional name for a column or table constraint.  If the
       constraint is violated, the constraint name is present in error messages,
-      so constraint names like "col must be positive" can be used to communicate
-      helpful constraint information to client applications.  If not specified,
-      the system generates a name.
+      so constraint names like <literal>"col must be positive"</> can be used
+      to communicate helpful constraint information to client applications.
+      (Double-quotes are required for constraint names that contain spaces.)
+      If not specified, the system generates a name.
      </para>
     </listitem>
    </varlistentry>