Clarify limit_length() for infinite vectors

[KoBeWi]

Closes #29714

The issue also mentions problems with multiplying, but you can't multiply infinity and 0, so the result makes sense. For limit_length() it's not obvious if you don't know the implementation.

[AThousandShips]

LGTM

[Repiteo]

Having explicitly undefined behavior feels a bit at-odds with the rest of the engine, but if we're keeping the scope to documentation then I can't complain

[Repiteo]

Thanks!