Merge and update #17282: Document unittest.main defaultTest argument.

In 3.4 defaultTest can also be a list (see issue 15132).

Doc/library/unittest.rst

@@ -1966,6 +1966,11 @@ Loading and running tests
       if __name__ == '__main__':
           unittest.main(verbosity=2)
 
+   The *defaultTest* argument is either the name of a single test or an
+   iterable of test names to run if no test names are specified via *argv*.  If
+   not specified or ``None`` and no test names are provided via *argv*, all
+   tests found in *module* are run.
+
    The *argv* argument can be a list of options passed to the program, with the
    first element being the program name.  If not specified or ``None``,
    the values of :data:`sys.argv` are used.