I didn't find anything about widget backups, so my question is, even if I assume it doesn't do that, I just want to have it sorted out:
Can marshmallow's new backup API save and restore homescreen widgets from apps? If or if not, why isn't it mentioned anywhere, is it that clear to anybody?
The backup API is for app data only. Home Screen icons and widgets are managed by the device's launcher app, which may choose to restore them if the device is reset.