Merge #14649: clarify DocTestSuite error when there are no docstrings.

Also adds tests to verify the documented behavior (which is probably a bug, as
indicated in the added comments).

Patch by Chris Jerdonek.

diff --cc Lib/doctest.py
Simple merge

diff --cc Lib/test/test_doctest.py
Simple merge

diff --cc Lib/test/test_zipimport_support.py
Simple merge