document that MergeAppend can now prune

[Amit Langote]

Hi.

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

commit 5220bb7533f9891b1e071da6461d5c387e8f7b09
Author: Heikki Linnakangas <heikki.linnakan...@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

diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index 6aa035188f..70f5fbe917 100644
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -3944,8 +3944,8 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate 
&gt;= DATE '2008-01-01';
     </para>
 
     <para>
-     Also, execution-time partition pruning currently only occurs for the
-     <literal>Append</literal> node type, not <literal>MergeAppend</literal>.
+     Execution-time partition pruning currently occurs for the
+     <literal>Append</literal> and <literal>MergeAppend</literal> node types.
     </para>
 
     <para>