[range] fixed #6212 (missing step in documentation for how to write a range adaptor)

[SVN r82314]

doc/reference/extending.qbk

@@ -218,6 +218,14 @@ operator|( const BidirectionalRng& r, detail::reverse_forwarder )
 }
 ``
 
+# Declare the adaptor itself (it is a variable of the tag type).
+``
+namespace
+{
+    const detail::reverse_forwarder reversed = detail::reverse_forwarder();
+}
+``
+
 [endsect]
 
 [section:method_3_2 Method 3.2: Implement a Range Adaptor with arguments]