In the current (3.3.3 and 3.4dev) Language Reference Manual, the section on the Raise statement fails to mention that the second expression can be None (per PEP-409/415) or the special behavior (suppressing a chained exception) that ensues.  Rather it explicitly states it can not be None:
 "...the second expression must be another exception class  or instance..."

It appears that although the Exceptions section of the Library Reference was updated when PEP-409/415 was implemented, the Language Reference was not.