document that MergeAppend can now prune

Hi.

It seems that the following commit missed updating ddl.sgml:

commit 5220bb7533f9891b1e071da6461d5c387e8f7b09
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date:   Thu Jul 19 13:49:43 2018 +0300

    Expand run-time partition pruning to work with MergeAppend

Attached patch fixes that.

Thanks,
Amit