bpo-27910: Update documentation of traceback module (GH-6116)

In the documentation for the traceback module, the definitions of functions
extract_tb(), format_list() and classmethod StackSummary.from_list()
mention the old style 4-tuples that these functions used to return or accept.

Since Python 3.5, however, they return or accept a FrameSummary object
instead of a 4-tuple, or a StackSummary object instead of a list of 4-tuples.

Co-Authored-By: Berker Peksag <berker.peksag@gmail.com>