http://docs.python.org/3/library/unittest.html#unittest.TestCase.assertWarns
"When used as a context manager, assertRaises() accepts"
"... is to perform additional checks on the exception raised"

New changeset 55dcf9e065be by Terry Jan Reedy in branch '3.3':
Issue #18573: Complete copy-paste from assertRaises entry to assertWarns entry.
http://hg.python.org/cpython/rev/55dcf9e065be

assertWarns is new in 3.2 and not in 2.7.
The sentence was copied from the assertRaises section but not changed.
Thanks for reporting.

What about the second line?
It doesn't catch any exception


utest.py

#!/usr/bin/env python3

import unittest

class Test(unittest.TestCase):

    def test_warning(self):
        import warnings

        with self.assertWarns(RuntimeWarning) as cm:
            raise ValueError('f')
            warnings.warn('f', RuntimeWarning)
        print(repr(cm.warning))


[guest@localhost py]$ python3 -m unittest utest
E
======================================================================
ERROR: test_warning (utest.Test)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "./utest.py", line 11, in test_warning
    raise ValueError('f')
ValueError: f

----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (errors=1)
[guest@localhost py]$


with commented raise

[guest@localhost py]$ python3 -m unittest utest
RuntimeWarning('f',)
.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK
[guest@localhost py]$


in the patch I copied terms from the first sentence of the paragraph

> What about the second line?
What second line?
> It doesn't catch any exception
It? I do not understand your point. Your example works as documented, including "any unexpected exception is an error".

Actually, that should be 'any exception' as none is expected.

Also, "additional checks on the exception raised:" should be 'warning caught'

New changeset 366beee880aa by Terry Jan Reedy in branch '3.3':
Issue #18573: More copy-paste fixes to assertWarns entry.
http://hg.python.org/cpython/rev/366beee880aa

> What second line?
the second line patched in the diff file