docs(exceptionHandler): document testing

Update src/ng/exceptionHandler.js

Here's an iniitla attempt at documenting how one might write a
test using $exceptionHandlerProvider. The key take-away is the use
of this pattern:

it(...

 module(...
   $exceptionHandlerProvider.mode('log');
 });

 inject(...
 );

});

src/ng/exceptionHandler.js

@@ -11,11 +11,12 @@
  * the browser console.
  *
  * In unit tests, if `angular-mocks.js` is loaded, this service is overridden by
- * {@link ngMock.$exceptionHandler mock $exceptionHandler}
+ * {@link ngMock.$exceptionHandler mock $exceptionHandler} which aids in testing.
  *
  * @param {Error} exception Exception associated with the error.
  * @param {string=} cause optional information about the context in which
  *       the error was thrown.
+ *
  */
 function $ExceptionHandlerProvider() {
   this.$get = ['$log', function($log){

src/ngMock/angular-mocks.js

@@ -202,6 +202,30 @@ angular.mock.$Browser.prototype = {
  * Mock implementation of {@link ng.$exceptionHandler} that rethrows or logs errors passed
  * into it. See {@link ngMock.$exceptionHandlerProvider $exceptionHandlerProvider} for configuration
  * information.
+ *
+ *
+ * <pre>
+ *   describe('$exceptionHandlerProvider', function() {
+ *
+ *     it('should capture log messages and exceptions', function() {
+ *
+ *       module(function($exceptionHandlerProvider) {
+ *         $exceptionHandlerProvider.mode('log');
+ *       });
+ *
+ *       inject(function($log, $exceptionHandler, $timeout) {
+ *         $timeout(function() { $log.log(1); });
+ *         $timeout(function() { $log.log(2); throw 'banana peel'; });
+ *         $timeout(function() { $log.log(3); });
+ *         expect($exceptionHandler.errors).toEqual([]);
+ *         expect($log.assertEmpty());
+ *         $timeout.flush();
+ *         expect($exceptionHandler.errors).toEqual(['banana peel']);
+ *         expect($log.log.logs).toEqual([[1], [2], [3]]);
+ *       });
+ *     });
+ *   });
+ * </pre>
  */
 
 angular.mock.$ExceptionHandlerProvider = function() {