Some other points to consider:  I don't think this belongs in the `for` statement docs at all.  Instead they should merely _note_ that what happens if a sequence being iterated over is mutated is up to the sequence iterator.

Then, e.g., the `list` docs should spell out the behavior currently (but, as already noted, incompletely) described.  From StackOverflow experience, I may be the only person in the world who knows that list behavior here is documented under the `for` statement - everyone else gives up after not finding a word about it in the `list` docs ;-)

`collections.deque` docs, in contrast, may wish to document the behavior of its iterators.  They in fact work hard to raise an exception if a size-changing mutation occurs during iteration, but that doesn't appear to be documented.  It's worth debating whether it should be.

Are there other mutable sequence types in the standard distribution?