Add a link in the doc.

doc/extending/cop.txt

+.. _cop:
 
 ====================================
 Implementing the arithmetic Ops in C

doc/extending/op.txt

@@ -161,8 +161,9 @@ following methods:
 
 At a bare minimum, a new Op must define ``make_node`` and ``perform``, which have no defaults.
 
-For more details, including the interface for providing a C
-implementation of ``perform()``, refer to the documentation for :ref:`op`.
+Also you can provide a :ref:`C implementation <cop>` of
+``perform()``. For other details refer to the documentation for
+:ref:`op`.
 
 
 Defining an Op: ``mul``