Improve assert expression documentation.

Per discussion on forum.
@quickfur: If it is undefined behaviour in /any/ build, then no check ever needs to be generated, even in non-release builds. I don't see which comment you mean.

@DanielGibson: My experience was different, and it is easier to stick to talking about undefined behaviour instead of assumptions. Besides, if 'assert' cannot be documented without saying 'assume', ... draw an analogy to 'immutable'.