bpo-42179: Clarify exception chaining

Doc/tutorial/errors.rst

@@ -301,8 +301,13 @@ This can be useful when you are transforming exceptions. For example::
 
 The expression following the :keyword:`from` must be either an exception or
 ``None``. Exception chaining happens automatically when an exception is raised
-inside an exception handler or :keyword:`finally` section. Exception chaining
-can be disabled by using ``from None`` idiom:
+inside an exception handler or :keyword:`finally` section, however in this case
+``__context__`` attribute of the exception is used. Chaining with
+``__context__`` works similarly to chaining with ``__cause__``,
+but when formatting exception traceback ``__cause__`` takes priority.
+For more information about chaining mechanics see :ref:`bltin-exceptions`.
+
+Exception chaining can be disabled by using ``from None`` idiom:
 
     >>> try:
     ...     open('database.sqlite')