Document the exponentiation operator as associating left to right.

Common mathematical convention is that exponentiation associates right to
left.  We aren't going to change the parser for this, but we could note
it in the operator's description.  (It's already noted in the operator
precedence/associativity table, but users might not look there.)
Per bug #13829 from Henrik Pauli.

doc/src/sgml/func.sgml

@@ -505,7 +505,7 @@
       <row>
        <entry> <literal>^</literal> </entry>
-       <entry>exponentiation</entry>
+       <entry>exponentiation (associates left to right)</entry>
        <entry><literal>2.0 ^ 3.0</literal></entry>
        <entry><literal>8</literal></entry>
       </row>