Doc: return 66c0185 to the release notes.

This reverts commit e1e83a7.

Per further discussion on pgsql-release, we wish to ship beta1 with
that feature, and patch the bug, rather than shipping beta1 with
that feature reverted.

Author: robertmhaas

doc/src/sgml/release-17.sgml

@@ -404,6 +404,17 @@ This can be disabled using server variable enable_group_by_reordering.
 </para>
 </listitem>
 
+<!--
+Author: David Rowley <drowley@postgresql.org>
+2024-03-25 [66c0185a3] Allow planner to use Merge Append to efficiently impleme
+-->
+
+<listitem>
+<para>
+Allow UNION (without ALL) to use MergeAppend (David Rowley)
+</para>
+</listitem>
+
 <!--
 Author: David Rowley <drowley@postgresql.org>
 2024-02-01 [9d1a5354f] Fix costing bug in MergeAppend