Propose better documentation for ActiveSupports next_week functionaility

[JonRowe]

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

[fxn]

Nice, the guide already explains it that way. Mind putting DateTime in fixed-width font please?

[JonRowe]

Sure thing @fxn

[fxn]

Awesome appreciate it.

[JonRowe]

Thanks :)