Provide module docstrings for the two main test drivers in importlib that

explain what they are for and how to use command-line arguments to tweak
semantics.

Lib/importlib/test/__main__.py

@@ -1,3 +1,9 @@
+"""Run importlib's test suite.
+
+Specifying the ``--builtin`` flag will run tests, where applicable, with
+builtins.__import__ instead of importlib.__import__.
+
+"""
 import importlib
 from importlib.test.import_ import util
 import os.path