Add documentation for the new notification support

pkg/analyzer_plugin/doc/tutorial/navigation.md

 # Providing Navigation
 
 Navigation information is used by clients to allow users to navigate to the
 location at which an identifier is defined.
 
 ## Implementation details
 
 Navigation information can be requested both by an `analysis.getNavigation`
 request and by a subscription. If the server has subscribed for navigation
 information in some set of files, the the plugin should send the information in
 an `analysis.navigation` notification whenever the information needs to be
 updated.
 
 When an `analysis.getNavigation` request is received, the method
 `handleAnalysisGetNavigation` will be invoked. This method is responsible for
 returning a response that contains the available navigation information.
 
-The easiest way to implement the method `handleAnalysisGetNavigation` is by
+When a notification needs to be sent, the method`sendNavigationNotification`

[scheglov, 2017/08/24 18:19:21]

Add a space between "method" and "`"?

+will be invoked. This method is responsible for sending the notification.
+
+The easiest way to add support for both the request and the notification is by
 adding the classes `NavigationMixin` and `DartNavigationMixin` (from
 `package:analyzer_plugin/plugin/navigation_mixin.dart`) to the list of mixins
 for your subclass of `ServerPlugin`. This will leave you with one abstract
 method that you need to implement: `getNavigationContributors`. That method is
 responsible for returning a list of `NavigationContributor`s. It is the
 navigation contributors that produce the actual navigation information. (Most
 plugins will only need a single navigation contributor.)
 
 To write a navigation contributor, create a class that implements
 `NavigationContributor`. The interface defines a single method named
@@ skipping 44 matching lines @@
 ```dart
 class MyPlugin extends ServerPlugin with NavigationMixin, DartNavigationMixin {
   // ...
 
   @override
   List<NavigationContributor> getNavigationContributors(String path) {
     return <NavigationContributor>[new MyNavigationContributor()];
   }
 }
 ```