docs(ngModel): describe $viewChangeListeners property

Justification:

I'm not sure if this is intended to be a private property or not, but
I think it might be a good idea to mention it in the docs, so that
people are aware of alternatives to adding redundant scope $watch functions.

At any rate, clarifying its public/private-ness would be a good thing to do,
as well as providing some warnings about ways it could be mistakenly abused.