Add contrib function references in the doc index

Backpatch to 9.3.
Idea from Craig Ringer

doc/src/sgml/pgbuffercache.sgml

@@ -12,6 +12,10 @@
   examining what's happening in the shared buffer cache in real time.
  </para>
 
+ <indexterm>
+  <primary>pg_buffercache_pages</primary>
+ </indexterm>
+
  <para>
   The module provides a C function <function>pg_buffercache_pages</function>
   that returns a set of records, plus a view