tests/cpydiff/core_function_userattr: Clarify, fill in cause and workaround.

tests/cpydiff/core_function_instancevar.py

-"""
-categories: Core,Functions
-description: Assign instance variable to function
-cause: Unknown
-workaround: Unknown
-"""
-def f():
-    pass
-
-f.x = 0
-print(f.x)

tests/cpydiff/core_function_userattr.py

+"""
+categories: Core,Functions
+description: User-defined attributes for functions are not supported
+cause: MicroPython is highly optimized for memory usage.
+workaround: Use external dictionary, e.g. ``FUNC_X[f] = 0``.
+"""
+def f():
+    pass
+
+f.x = 0
+print(f.x)