doc: improve PG 14 relnote item about adding btree pages to FSM

Wording confirmed by Peter Geoghegan.

Backpatch-through: 14 only

doc/src/sgml/release-14.sgml

@@ -720,13 +720,13 @@ Author: Peter Geoghegan <pg@bowt.ie>
 -->
 
        <para>
-        Allow vacuum to eagerly add newly deleted btree pages to the free
+        Allow vacuum to more eagerly add deleted btree pages to the free
         space map (Peter Geoghegan)
        </para>
 
        <para>
-        Previously vacuum could only place preexisting deleted pages in
-        the free space map.
+        Previously vacuum could only add pages to the free space map
+        that were marked as deleted by previous vacuums.
        </para>
       </listitem>