Group the functions in the hash reference to make it a little clearer.

[SVN r37567]

hash/doc/ref.xml

@@ -406,6 +406,8 @@ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
         </method>
       </struct-specialization>
 
+      <free-function-group name="Support functions (Boost extension).">
+
       <!--
         boost::hash_combine
         -->
@@ -500,6 +502,10 @@ for(; first != last; ++first)
         </para></throws>
       </overloaded-function>
 
+      </free-function-group>
+
+      <free-function-group name="Overloadable hash implementation (Boost extension).">
+
       <!--
         boost::hash_value - integers
         -->
@@ -758,6 +764,8 @@ return seed;</programlisting></entry>
           </informaltable>
         </returns>
       </overloaded-function>
+
+      </free-function-group>
     </namespace>
   </header>
 </library-reference>