Add index entries for more functions

Also, move index entries into the tables, closer to the function description,
for easier editing in the future.  Resort some tables to be more alphabetical.
Remove the entries for count, max, min, and sum in the tutorial area, because
that was felt to be confusing.

Thom Brown

doc/src/sgml/query.sgml

@@ -642,12 +642,6 @@ SELECT *
    </indexterm>
 
    <para>
-    <indexterm><primary>average</primary></indexterm>
-    <indexterm><primary>count</primary></indexterm>
-    <indexterm><primary>max</primary></indexterm>
-    <indexterm><primary>min</primary></indexterm>
-    <indexterm><primary>sum</primary></indexterm>
-
     Like  most  other relational database products,
     <productname>PostgreSQL</productname> supports
     <firstterm>aggregate functions</>.