Propose better documentation for ActiveSupports `next_week` functionaility

Update documentation for `next_week(:day)` to better reflect implementation.

Asking a Date(Time) for `next_week` is the equivalent of saying 'Give me day X in next week' but the original documentation makes it seem like the parameter is the start of next week.

This PR updates the documentation to reflect the reality, e.g. asking for `next_week(:wednesday)` is the equivalent of the english statement 'next wednesday' or 'the wednesday in next week'.

This will clear up misunderstandings like #9568