docs(select): improve formatting and wording

The part about using `select as` and `track by` together was hard to read.
And it wasn't clear what happens if they're used together.

Closes #10409

Author: thorn0

src/ng/directive/select.js

@@ -13,14 +13,15 @@ var ngOptionsMinErr = minErr('ngOptions');
  *
  * The `ngOptions` attribute can be used to dynamically generate a list of `<option>`
  * elements for the `<select>` element using the array or object obtained by evaluating the
- * `ngOptions` comprehension_expression.
+ * `ngOptions` comprehension expression.
  *
  * In many cases, `ngRepeat` can be used on `<option>` elements instead of `ngOptions` to achieve a
- * similar result. However, the `ngOptions` provides some benefits such as reducing memory and
+ * 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 `select as`. `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.
+ * 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.
  *
  * 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`
@@ -35,28 +36,51 @@ var ngOptionsMinErr = minErr('ngOptions');
  * array of objects. See an example [in this jsfiddle](http://jsfiddle.net/qWzTb/).
  * </div>
  *
- * ## `select as`
+ * ## `select` **`as`**
  *
- * Using `select as` will bind the result of the `select as` expression to the model, but
+ * Using `select` **`as`** will bind the result of the `select` expression to the model, but
  * the value of the `<select>` and `<option>` html elements will be either the index (for array data sources)
- * or property name (for object data sources) of the value within the collection. If a `track by` expression
+ * or property name (for object data sources) of the value within the collection. If a **`track by`** expression
  * is used, the result of that expression will be set as the value of the `option` and `select` elements.
  *
- * ### `select as` with `track by`
  *
- * Using `select as` together with `track by` is not recommended. Reasoning:
+ * ### `select` **`as`** and **`track by`**
+ *
+ * <div class="alert alert-warning">
+ * Do not use `select` **`as`** and **`track by`** in the same expression. They are not designed to work together.
+ * </div>
+ *
+ * Consider the following example:
+ *
+ * ```html
+ * <select ng-options="item.subItem as item.label for item in values track by item.id" ng-model="selected">
+ * ```
+ *
+ * ```js
+ * $scope.values = [{
+ *   id: 1,
+ *   label: 'aLabel',
+ *   subItem: { name: 'aSubItem' }
+ * }, {
+ *   id: 2,
+ *   label: 'bLabel',
+ *   subItem: { name: 'bSubItem' }
+ * }];
+ *
+ * $scope.selected = { name: 'aSubItem' };
+ * ```
+ *
+ * With the purpose of preserving the selection, the **`track by`** expression is always applied to the element
+ * of the data source (to `item` in this example). To calculate whether an element is selected, we do the
+ * following:
+ *
+ * 1. Apply **`track by`** to the elements in the array. In the example: `[1, 2]`
+ * 2. Apply **`track by`** to the already selected value in `ngModel`.
+ *    In the example: this is not possible as **`track by`** refers to `item.id`, but the selected
+ *    value from `ngModel` is `{name: 'aSubItem'}`, so the **`track by`** expression is applied to
+ *    a wrong object, the selected element can't be found, `<select>` is always reset to the "not
+ *    selected" option.
  *
- * - Example: &lt;select ng-options="item.subItem as item.label for item in values track by item.id" ng-model="selected"&gt;
- *   values: [{id: 1, label: 'aLabel', subItem: {name: 'aSubItem'}}, {id: 2, label: 'bLabel', subItem: {name: 'bSubItem'}}],
- *   $scope.selected = {name: 'aSubItem'};
- * - track by is always applied to `value`, with the purpose of preserving the selection,
- *   (to `item` in this case)
- * - to calculate whether an item is selected we do the following:
- *   1. apply `track by` to the values in the array, e.g.
- *      In the example: [1,2]
- *   2. apply `track by` to the already selected value in `ngModel`:
- *      In the example: this is not possible, as `track by` refers to `item.id`, but the selected
- *      value from `ngModel` is `{name: aSubItem}`.
  *
  * @param {string} ngModel Assignable angular expression to data-bind to.
  * @param {string=} name Property name of the form under which the control is published.