Small run-time pruning doc fix

Before 5220bb7533f a note in ddl.sgml used to mention that run-time
pruning was only implemented for Append. When we got MergeAppend
support the commit updated this to mention 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>

The attached patch updates this to mention that ModifyTable is
unsupported which I think makes the above fragment make sense again.

-- 
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services