docs(navBackButton): clarify custom back button action
fix(toggle): fix ngChange being reported before model changes

Closes #1349, #1741

BREAKING CHANGE:

ion-toggle no longer has an isolate scope.
This will break your toggle only if you were relying upon the toggle
having an isolate scope: if you were referencing `$parent.value` as
the ng-disabled attribute, for example.

Change your code from this:

<ion-toggle ng-disabled="{{$parent.isDisabled}}"></ion-toggle>

To this:

<ion-toggle ng-disabled="{{isDisabled}}"></ion-toggle>