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

Request Type: docs

How to reproduce: 

Component(s): scope

Impact: large

Complexity: small

This issue is related to: 

**Detailed Description:**

During the first digest loop after registering a $watch
the listener always run, so the example about the $digest cycles in the $rootScope docs was incorrect
(here's a plunk to prove it:
http://plnkr.co/edit/NRM4UIp9rk28VbylAtvl).
Also, we can make it clearer that if the value doesn't change,
the listener doesn't run on subsequent $digest loops.

**Other Comments:**