Document debug_perform().

doc/extending/op.txt

@@ -266,6 +266,20 @@ Optional methods or attributes
    As done in the Alloc op, you can return False only in some cases by
    analyzing the graph from the node parameter.
 
+.. function:: debug_perform(node, inputs, output_storage)
+
+   Undefined by default.
+
+   If you define this function then it will be used instead of C code
+   or perform() to do the computation while debugging (currently
+   DebugMode, but others amy also use it in the future).  It has the
+   same signature and contract as :func:`perform`.
+
+   This enables ops that don't have a deterministic output for
+   efficiency reasons to make their output determinisitic or do fix
+   problems that they encounter with DebugMode.  If your op doesn't
+   have any problems, don't implement this.
+
 If you want your op to work with gradient.grad() you also need to
 implement the functions described below.