doc: Small run-time pruning doc fix

A note in ddl.sgml used to mention that run-time pruning was only
implemented for Append.  When we got MergeAppend support, this was
updated to mention that MergeAppend is supported too.  This is
slightly weird as it's not all that obvious what exactly isn't
supported when we mention:

    <para>
     Both of these behaviors are likely to be changed in a future release
     of <productname>PostgreSQL</productname>.
    </para>

This patch updates this to mention that ModifyTable is unsupported,
which makes the above fragment make sense again.

Author: David Rowley <david.rowley@2ndquadrant.com>

doc/src/sgml/ddl.sgml

@@ -3944,8 +3944,10 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2008-01-01';
     </para>
 
     <para>
-     Execution-time partition pruning currently occurs for the
+     Execution-time partition pruning currently only occurs for the
      <literal>Append</literal> and <literal>MergeAppend</literal> node types.
+     It is not yet implemented for the <literal>ModifyTable</literal> node
+     type.
     </para>
 
     <para>