tests/cpydiff/core_arguments: Fill in cause/workaround.

tests/cpydiff/core_arguments.py

 """
 categories: Core
-description: Error messages may display incorrect argument counts
-cause: Unknown
-workaround: Unknown
+description: Error messages for methods may display unexpected argument counts
+cause: MicroPython counts "self" as an argument.
+workaround: Interpret error messages with the information above in mind.
 """
 try:
     [].append()