docs(guide/expression): fix docs re $window

The documentation on context is incorrect and misleading:

1. "Angular expressions must use $window explicitly to refer to the global
   `window` object": expressions cannot access `$window`
1. The example doesn't actually attempt to use $window in a expression. It's in a
   function called from an expression, which incorrectly implies to readers that:
  1. functions ARE expressions
  1. functions called by expressions can't access `window`

Here's [a plunkr](http://plnkr.co/edit/Gd4xAV?p=preview) to make both these issues clear.

This change fixes the errors and informs the reader about Angular's `$window` etc services,
and adds an explicit example of an expression not being able to access `window`.

docs/content/guide/expression.ngdoc

@@ -95,16 +95,18 @@ You can try evaluating different expressions here:
 Angular does not use JavaScript's `eval()` to evaluate expressions. Instead Angular's
 {@link ng.$parse $parse} service processes these expressions.
 
-Unlike JavaScript, where names default to global `window` properties, Angular expressions must use
-{@link ng.$window `$window`} explicitly to refer to the global `window` object. For example, if you
-want to call `alert()` in an expression you must use `$window.alert()`. This restriction is
-intentional. It prevents accidental access to the global state – a common source of subtle bugs.
+Angular expressions do not have access to global variables like `window`, `document` or `location`.
+This restriction is intentional. It prevents accidental access to the global state – a common source of subtle bugs.
+
+Instead use services like `$window` and `$location` in functions called from expressions. Such services
+provide mockable access to globals.
 
 <example>
   <file name="index.html">
     <div class="example2" ng-controller="Cntl1">
       Name: <input ng-model="name" type="text"/>
       <button ng-click="greet()">Greet</button>
+      <button ng-click="window.alert('Should not see me')">Won't greet</button>
     </div>
   </file>