Add index entries for function volatilities.

doc/src/sgml/xfunc.sgml

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-$PostgreSQL: pgsql/doc/src/sgml/xfunc.sgml,v 1.95 2005/01/08 22:13:36 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/xfunc.sgml,v 1.96 2005/01/14 21:46:09 jurka Exp $
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  <sect1 id="xfunc">
@@ -772,6 +772,15 @@ CREATE FUNCTION test(int, int) RETURNS int
     <primary>volatility</primary>
     <secondary>functions</secondary>
    </indexterm>
+   <indexterm zone="xfunc-volatility">
+    <primary>VOLATILE</primary>
+   </indexterm>
+   <indexterm zone="xfunc-volatility">
+    <primary>STABLE</primary>
+   </indexterm>
+   <indexterm zone="xfunc-volatility">
+    <primary>IMMUTABLE</primary>
+   </indexterm>
 
    <para>
     Every function has a <firstterm>volatility</> classification, with