Document first day of week for time intervals

Contrary to ISO 8601 and RFC 3339, Alertmanager regards Sunday as the first day of the week rather than Monday, so weekends cannot be written as saturday:sunday, and "every day" would have to be written as sunday:saturday rather than monday:sunday.

I think it would be even better if the "first" day of the week was an implementation detail, and ranges like friday:monday worked intuitively. I'm happy to implement this if there is agreement.