Minor enhancement to the plugin tutorials

pkg/analyzer_plugin/doc/tutorial/navigation.md

Index: pkg/analyzer_plugin/doc/tutorial/navigation.md
diff --git a/pkg/analyzer_plugin/doc/tutorial/navigation.md b/pkg/analyzer_plugin/doc/tutorial/navigation.md
index d267241aa017985c6c96890bbc8c13d512a3d422..fedae23a7069e0c1d55078c37cdec755374f07b0 100644
--- a/pkg/analyzer_plugin/doc/tutorial/navigation.md
+++ b/pkg/analyzer_plugin/doc/tutorial/navigation.md
@@ -30,6 +30,9 @@ To write a navigation contributor, create a class that implements
 describes the region of the file for which navigation is being requested and a
 `NavigationCollector` through which navigation information is to be added.
 
+If you mix in the class `DartNavigationMixin`, then the request will be an
+instance of `DartNavigationRequest`, which also has analysis results.
+
 ## Example
 
 Start by creating a class that implements `NavigationContributor`, then