Some clarifications added

I added a few tips to help noobs get started with registering helper functions and validations.  Prior to my changes, the docs did not fully demonstrate where to add these rules, so the user was left guessing and googling for possible solutions.
I agree it should be made clear that whatever is set within a mutator is the *true value*. But, to say "because of this, consider using this type of implementation" is just something I wouldn't want to see in a doc, since I don't necessarily agree with the implementation shown here; it varies so much between applications there is no right answer. 

Mutators are a pretty advanced topic, so if someone requires this much hand holding to save a variable I would ask them to reconsider if mutators are what they want.