relnotes: clarify that PG 15 multi-xid is advanced more

Reported-by: Peter Geoghegan

Discussion: https://postgr.es/m/CAH2-Wzk-oOpKObMKJ=Df4WBERX0ja14ymu3h5JrDt_FtmvH1yQ@mail.gmail.com

Backpatch-through: 15 only

Author: bmomjian

doc/src/sgml/release-15.sgml

@@ -994,7 +994,8 @@ Author: Peter Geoghegan <pg@bowt.ie>
       <listitem>
        <para>
         Allow <link linkend="routine-vacuuming">vacuum</link> to be more
-        aggressive in setting the oldest frozenxid (Peter Geoghegan)
+        aggressive in setting the oldest frozen and multi transaction id
+        (Peter Geoghegan)
        </para>
       </listitem>