Bug#648633: o properly document non preserved variables

[Regid Ichira]

  Did I understand correctly that not preserving changes across reboots to the UTC variable, and possibly others, will not be changed in the future?  If so, please
properly document that.  I think the first place such a thing should be documented is in the related file, which is currently /etc/default/rcS.  
  As far as I can tell, the current situation is not documented at all.