Minor enhancement to the plugin tutorials

pkg/analyzer_plugin/doc/tutorial/completion.md

Index: pkg/analyzer_plugin/doc/tutorial/completion.md
diff --git a/pkg/analyzer_plugin/doc/tutorial/completion.md b/pkg/analyzer_plugin/doc/tutorial/completion.md
index 883921c96e86aea214d439739a4f90d010ecbfd9..3776713a7eb93f3d85ec4b623a3f2f535458688f 100644
--- a/pkg/analyzer_plugin/doc/tutorial/completion.md
+++ b/pkg/analyzer_plugin/doc/tutorial/completion.md
@@ -45,6 +45,9 @@ To write a completion contributor, create a class that implements
 describes the where completions are being requested and a `CompletionCollector`
 through which suggestions are to be added.
 
+If you mix in the class `DartCompletionMixin`, then the request will be an
+instance of `DartCompletionRequest`, which also has analysis results.
+
 ## Example
 
 Start by creating a class that implements `CompletionContributor`, then