Fix documentation for cppclass static methods.

Closes #1438.

docs/src/userguide/wrapping_CPlusPlus.rst

@@ -612,6 +612,8 @@ If the Rectangle class has a static member:
 you can declare it using the Python @staticmethod decorator, i.e.::
 
     cdef extern from "Rectangle.h" namespace "shapes":
+        cdef cppclass Rectangle:
+            ...
             @staticmethod
             void do_something()