docs(ngModel): fix grammar and improve clarity

Closes #4291

Author: jdavisp3

src/ng/directive/input.js

@@ -1145,18 +1145,18 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
  * @element input
  *
  * @description
- * Is a directive that tells Angular to do two-way data binding. It works together with `input`,
- * `select`, `textarea` and even custom form controls that use {@link ng.directive:ngModel.NgModelController
- * NgModelController} exposed by this directive.
+ * The `ngModel` directive binds an `input`,`select`, `textarea` (or custom form control) to a
+ * property on the scope using {@link ng.directive:ngModel.NgModelController NgModelController},
+ * which is created and exposed by this directive.
  *
  * `ngModel` is responsible for:
  *
- * - binding the view into the model, which other directives such as `input`, `textarea` or `select`
- *   require,
- * - providing validation behavior (i.e. required, number, email, url),
- * - keeping state of the control (valid/invalid, dirty/pristine, validation errors),
- * - setting related css class onto the element (`ng-valid`, `ng-invalid`, `ng-dirty`, `ng-pristine`),
- * - register the control with parent {@link ng.directive:form form}.
+ * - Binding the view into the model, which other directives such as `input`, `textarea` or `select`
+ *   require.
+ * - Providing validation behavior (i.e. required, number, email, url).
+ * - Keeping the state of the control (valid/invalid, dirty/pristine, validation errors).
+ * - Setting related css classes on the element (`ng-valid`, `ng-invalid`, `ng-dirty`, `ng-pristine`).
+ * - Registering the control with its parent {@link ng.directive:form form}.
  *
  * Note: `ngModel` will try to bind to the property given by evaluating the expression on the
  * current scope. If the property doesn't already exist on this scope, it will be created