Fix documentation for run-time partition pruning

Since 5220bb7, not only Append, but also MergeAppend support the
operation.

Author: Amit Langote
Discussion: https://postgr.es/m/59d8eb92-4536-c44e-54e2-305b9b3d8eb7@lab.ntt.co.jp

doc/src/sgml/ddl.sgml

@@ -3944,8 +3944,8 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= 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>