Clarify threading documentation: atomic list.append

[mdickinson]

Some confusion was reported with regard to this sentence in the threading docs:

For example, list.append may happen to be atomic in current versions of CPython, but there’s no guarantee that that will remain the case, and you may find that your code is actually working with a subclass of list (like TraitList) for which append is not thread-safe.

The sentence could be broken up, and it should be rephrased to make clear that "working" does not mean "functioning correctly", but rather something along the lines of "dealing with".