LibRef 2.2.6.4 Mutable Sequence Types footnote (2) 
(current version)to S.extend(x) says "Raises an 
exception when x is not a list object."  Similarly,
>>> a.extend.__doc__
'L.extend(list) -- extend list by appending list elements'

However, x need only be iterable.  For instance (2.2.1)

>>> a.extend('ab')
>>> a
[1, 2, 3, 'a', 'b']
>> from __future__ import generators
>>> def f(n):
...   for i in range(n): yield i
...
>>> a.extend(f(3))
>>> a
[1, 2, 3, 'a', 'b', 0, 1, 2]

Error message has already been updated:
>>> a.extend(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: list.extend() argument must be iterable

Question: footnote 2 also says "The extend() method 
is experimental".  Still true after two additional 
releases, or time to delete this?

The continuation "and not supported by mutable 
sequence types other than lists. " seems rather moot 
(and delete-able) unless and until another m.s.t is 
added.  

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Fixed.

See listobject.c revision: 2.143 and 2.103.6.5.
and libstdtypes.tex revision: 1.115 and 1.80.6.17.

Closing bug.