docs(input): update $parsers NgModelController doc

Make it clear that the result of the $parsers pipeline is what goes to the model value.

Closes #5708

src/ng/directive/input.js

@@ -878,8 +878,9 @@ var VALID_CLASS = 'ng-valid',
  * @property {*} $modelValue The value in the model, that the control is bound to.
  * @property {Array.<Function>} $parsers Array of functions to execute, as a pipeline, whenever
        the control reads value from the DOM.  Each function is called, in turn, passing the value
-       through to the next. Used to sanitize / convert the value as well as validation.
-       For validation, the parsers should update the validity state using
+       through to the next. The last return value is used to populate the model.
+       Used to sanitize / convert the value as well as validation. For validation,
+       the parsers should update the validity state using
        {@link ngModel.NgModelController#$setValidity $setValidity()},
        and return `undefined` for invalid values.