tests/cpydiff: Update subclassing Exception case and give work-around.

tests/cpydiff/types_exception_subclassinit.py

 """
 categories: Types,Exception
-description: Exception.__init__ raises TypeError if overridden and called by subclass
-cause: Unknown
-workaround: Unknown
+description: Exception.__init__ method does not exist.
+cause: Subclassing native classes is not fully supported in MicroPython.
+workaround: Call using ``super()`` instead::
+
+class A(Exception):
+    def __init__(self):
+        super().__init__()
 """
 class A(Exception):
     def __init__(self):