Merge visibility map crash-safety into index-only scan release note

item, per Robert Haas.

doc/src/sgml/release-9.2.sgml

@@ -413,7 +413,8 @@
        <para>
         This is often called "index-only scans" or "covering indexes".
         This is possible for heap pages with exclusively all-visible
-        tuples, as reported by the visibility map.
+        tuples, as reported by the visibility map.  The visibility map was
+        made crash-safe as a necessary part of implementing this feature.
        </para>
       </listitem>
 
@@ -539,17 +540,6 @@
        </para>
       </listitem>
       
-      <listitem>
-       <para>
-        Make the visibility map crash-safe (Robert Haas, Noah Misch)
-       </para>
-       
-       <para>
-        This helps vacuum be more efficient, and is necessary for
-        index-only scans.
-       </para>
-      </listitem>
-      
       <listitem>
        <para>
         Improve PowerPC and Itanium spinlock performance (Manabu Ori,