docs(form): clarify what $setPristine does

Author: Narretz

src/ng/directive/form.js

@@ -252,9 +252,11 @@ function FormController(element, attrs, $scope, $animate, $interpolate) {
    * @description
    * Sets the form to its pristine state.
    *
-   * This method can be called to remove the 'ng-dirty' class and set the form to its pristine
-   * state (ng-pristine class). This method will also propagate to all the controls contained
-   * in this form.
+   * This method sets the form's `$pristine` state to true, the `$dirty` state to false, removes
+   * the `ng-dirty` class and adds the `ng-pristine` class. Additionally, it sets the `$submitted`
+   * state to false.
+   *
+   * This method will also propagate to all the controls contained in this form.
    *
    * Setting a form back to a pristine state is often useful when we want to 'reuse' a form after
    * saving or resetting it.