Create temporary pg.auto.conf file with the same permissions as the original

Commit 0574279 set the file permissions to 0600 rather than the user's
umask, but if initdb was executed with -g/--allow-group-access, the
file is maintained with 0640, so we'll just maintain the existing
permssions.

doc/appendix-release-notes.xml

@@ -136,6 +136,13 @@
             </para>
           </listitem>
 
+          <listitem>
+            <para>
+              Ensure <filename>postgresql.auto.conf</filename> is created with
+              correct permissions (PostgreSQL 12 and later).
+            </para>
+          </listitem>
+
         </itemizedlist>
       </para>
     </sect2>