Document calling extend methods from C/C++

Note that it's necessary to use the naming scheme we already document
to do this.

Fixes https://sourceforge.net/p/swig/bugs/889/

Doc/Manual/SWIG.html

@@ -3016,6 +3016,11 @@ typedef struct Vector {
 </pre>
 </div>
 
+<p>
+You'll also need to use these names if you want to directly call methods added
+using <tt>%extend</tt> from other C/C++ code.
+</p>
+
 <p>
 The name used for %extend should be the name of the struct and not the name of any typedef to the struct.
 For example: