docs(form): add info about pending prop and class

Closes #12704

Author: joakimbl

src/ng/directive/form.js

@@ -25,6 +25,7 @@ function nullFormRenameControl(control, name) {
  * @property {boolean} $dirty True if user has already interacted with the form.
  * @property {boolean} $valid True if all of the containing forms and controls are valid.
  * @property {boolean} $invalid True if at least one containing control or form is invalid.
+ * @property {boolean} $pending True if at least one containing control or form is pending.
  * @property {boolean} $submitted True if user has submitted the form even if its invalid.
  *
  * @property {Object} $error Is an object hash, containing references to controls or
@@ -329,6 +330,7 @@ function FormController(element, attrs, $scope, $animate, $interpolate) {
  * # CSS classes
  *  - `ng-valid` is set if the form is valid.
  *  - `ng-invalid` is set if the form is invalid.
+ *  - `ng-pending` is set if the form is pending.
  *  - `ng-pristine` is set if the form is pristine.
  *  - `ng-dirty` is set if the form is dirty.
  *  - `ng-submitted` is set if the form was submitted.