Documentation for `ExactSizeIterator` does not make clear that `len()` should change with iteration

[alice-i-cecile]

Reading between the lines on ExactSizeIterator (and investigating other implementations), it appears that the value of the returned len() should change as iteration occurs.

This is very subtle, and was missed by our entire team, see bevyengine/bevy#5149.

This information should be discussed prominently in the docs.

[Rageking8]

@rustbot label A-docs