docs: document reset feature

[harry48225]

Adds documentation for the reset feature

step_url = kwargs.get('step', None)
if step_url is None:
    if 'reset' in self.request.GET:
        self.storage.reset()
        self.storage.current_step = self.steps.first
    query_string = '?%s' % self.request.GET.urlencode() if self.request.GET else ''
    return redirect(self.get_step_url(self.steps.current) + query_string)