Add a comment to explain the workaround

Author: HyukjinKwon

python/pyspark/util.py

@@ -35,6 +35,10 @@ def _exception_message(excp):
     True
     """
     if isinstance(excp, Py4JJavaError):
+        # 'Py4JJavaError' doesn't contain the stack trace available on the Java side in 'message'
+        # attribute in Python 2. We should call 'str' function on this exception in general but
+        # 'Py4JJavaError' has an issue about addressing non-ascii strings. So, here we work
+        # around by the direct call, '__str__()'. Please see SPARK-23517.
         return excp.__str__()
     if hasattr(excp, "message"):
         return excp.message