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

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

diff --cc Doc/library/unittest.rst
index 46d303ab63999a3f947be91f6668b5682d3d9b79,fcfd514d01501cbcd0926a93bc16610ca685e024..3f3012722ada29cdbd105a1074faf3b6cee1504b
--- 1/Doc/library/unittest.rst
--- 2/Doc/library/unittest.rst
+++ b/Doc/library/unittest.rst
@@@ -1966,6 -1801,10 +1966,11 @@@ Loading and running test
        if __name__ == '__main__':
            unittest.main(verbosity=2)
  
 -   The *defaultTest* argument is the name of the test 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 *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.