docs($rootScope): fix incorrect docs and make them clearer

During the first $digest loop after registering a $watch
the listener always run, so the example was incorrect
(here's a plunk proving it:
http://plnkr.co/edit/NRM4UIp9rk28VbylAtvl).
Also, I'm making it clearer that if the value doesn't change,
the listener doesn't get called on subsequent $digest loops.