docs(select): provide more info and example about non-string options

See #11890

Author: petebacondarwin

src/ng/directive/select.js

@@ -112,9 +112,7 @@ var SelectController =
  * ngOptions} to achieve a similar result. However, `ngOptions` provides some benefits such as reducing
  * memory and increasing speed by not creating a new scope for each repeated instance, as well as providing
  * more flexibility in how the `<select>`'s model is assigned via the `select` **`as`** part of the
- * comprehension expression. `ngOptions` should be used when the `<select>` model needs to be bound
- * to a non-string value. This is because an option element can only be bound to string values at
- * present.
+ * comprehension expression.
  *
  * When an item in the `<select>` menu is selected, the array element or object property
  * represented by the selected option will be bound to the model identified by the `ngModel`
@@ -127,6 +125,51 @@ var SelectController =
  * be nested into the `<select>` element. This element will then represent the `null` or "not selected"
  * option. See example below for demonstration.
  *
+ * <div class="alert alert-info">
+ * The value of a `select` directive used without `ngOptions` is always a string.
+ * When the model needs to be bound to a non-string value, you must either explictly convert it
+ * using a directive (see example below) or use `ngOptions` to specify the set of options.
+ * This is because an option element can only be bound to string values at present.
+ * </div>
+ *
+ * ### Example (binding `select` to a non-string value)
+ *
+ * <example name="select-with-non-string-options" module="nonStringSelect">
+ *   <file name="index.html">
+ *     <select ng-model="model.id" convert-to-number>
+ *       <option value="0">Zero</option>
+ *       <option value="1">One</option>
+ *       <option value="2">Two</option>
+ *     </select>
+ *     {{ model }}
+ *   </file>
+ *   <file name="app.js">
+ *     angular.module('nonStringSelect', [])
+ *       .run(function($rootScope) {
+ *         $rootScope.model = { id: 2 };
+ *       })
+ *       .directive('convertToNumber', function() {
+ *         return {
+ *           require: 'ngModel',
+ *           link: function(scope, element, attrs, ngModel) {
+ *             ngModel.$parsers.push(function(val) {
+ *               return parseInt(val, 10);
+ *             });
+ *             ngModel.$formatters.push(function(val) {
+ *               return '' + val;
+ *             });
+ *           }
+ *         };
+ *       });
+ *   </file>
+ *   <file name="protractor.js" type="protractor">
+ *     it('should initialize to model', function() {
+ *       var select = element(by.css('select'));
+ *       expect(element(by.model('model.id')).$('option:checked').getText()).toEqual('Two');
+ *     });
+ *   </file>
+ * </example>
+ *
  */
 var selectDirective = function() {