test.support.captured_output is not covered in the online documents:
http://docs.python.org/3.1/library/test.html
http://docs.python.org/dev/py3k/library/test.html

However, it does have a docstring in "C:\Python31\Lib\test\support.py" (see below). The current example for "captured_output" does not work. Looks like someone moved it from "captured_stdout" but did not fully update it. Note the old example still references "captured_stdout". 

# Here's the current code in "support.py":

@contextlib.contextmanager
def captured_output(stream_name):
    """Run the 'with' statement body using a StringIO object in place of a
    specific attribute on the sys module.
    Example use (with 'stream_name=stdout')::

       with captured_stdout() as s:
           print("hello")
       assert s.getvalue() == "hello"
    """
    import io
    orig_stdout = getattr(sys, stream_name)
    setattr(sys, stream_name, io.StringIO())
    try:
        yield getattr(sys, stream_name)
    finally:
        setattr(sys, stream_name, orig_stdout)

def captured_stdout():
    return captured_output("stdout")

# Example for captured_output should now be:

       with captured_output("stdout") as s:
           print("hello")
       assert s.getvalue() == "hello"

# It would be nice to reconcile the online doc versus the docstrings, since it confusing and makes me confused whether captured_stdout is deprecated.

I don't see why the example wouldn't work anymore.  It's just in the wrong function's docs :)

New changeset 459e2c024420 by Ezio Melotti in branch '2.7':
#7960: fix docstrings for captured_output and captured_stdout.
http://hg.python.org/cpython/rev/459e2c024420

New changeset c2126d89c29b by Ezio Melotti in branch '3.1':
#7960: fix docstrings for captured_output and captured_stdout.
http://hg.python.org/cpython/rev/c2126d89c29b

New changeset 18a192ae6db9 by Ezio Melotti in branch '3.2':
#7960: merge with 3.1.
http://hg.python.org/cpython/rev/18a192ae6db9

New changeset 7517add4aec9 by Ezio Melotti in branch 'default':
#7960: merge with 3.2.
http://hg.python.org/cpython/rev/7517add4aec9

I fixed the docstring, however I think captured_output should be renamed _captured_output, since it only works with sys.stdout/in/err and there are already 3 other functions (in 3.2/3.3) that use captured_output to replace the 3 std* streams.  There's no reason to document and use it elsewhere.
Georg, what do you think?

New changeset ec35f86efb0d by Ezio Melotti in branch 'default':
Merge with 3.2 and also remove captured_output from __all__ (see #7960).
http://hg.python.org/cpython/rev/ec35f86efb0d