Doc: improve documentation about exponentiation operator.

Now that we're not having to wedge this into the straitjacket of
the old operator table format, we can add another example to
clarify the point about left-to-right associativity.

Per suggestion from mdione at grulic.org.ar.

https://postgr.es/m/162661954599.693.13700316547731859171@wrigleys.postgresql.org

doc/src/sgml/func.sgml

@@ -988,16 +988,23 @@ repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
         <returnvalue>double precision</returnvalue>
        </para>
        <para>
-        Exponentiation (unlike typical mathematical practice, multiple uses of
-        <literal>^</literal> will associate left to right)
+        Exponentiation
        </para>
        <para>
         <literal>2 ^ 3</literal>
         <returnvalue>8</returnvalue>
        </para>
+       <para>
+        Unlike typical mathematical practice, multiple uses of
+        <literal>^</literal> will associate left to right by default:
+       </para>
        <para>
         <literal>2 ^ 3 ^ 3</literal>
         <returnvalue>512</returnvalue>
+       </para>
+       <para>
+        <literal>2 ^ (3 ^ 3)</literal>
+        <returnvalue>134217728</returnvalue>
        </para></entry>
       </row>