release notes: add item missed in 9.2.5 release

Item is "Prevent errors in WAL replay due to references to uninitialized
empty pages".

Report and text by Andres Freund

Backpatch through 9.2.

doc/src/sgml/release-9.2.sgml

@@ -1178,6 +1178,13 @@
      </para>
     </listitem>
 
+    <listitem>
+     <para>
+      Prevent errors in WAL replay due to references to uninitialized empty
+      pages (Andres Freund)
+     </para>
+    </listitem>
+
     <listitem>
      <para>
       Fix <command>REINDEX TABLE</> and <command>REINDEX DATABASE</>