Minor enhancement to the plugin tutorials

pkg/analyzer_plugin/doc/tutorial/completion.md

 # Providing Code Completions
 
 A code completion is used by clients to provide a set of possible completions to
 partially entered code. Completions are intended to address two use cases: to
 help users enter code with less effort and to help users discover the behavior
 of an object.
 
 For example, if the user has typed `o.toSt` and then requested completions, one
 suggestion might be `toString`.
 
@@ skipping 27 matching lines @@
 responsible for returning a list of `CompletionContributor`s. It is the
 completion contributors that produce the actual completion suggestions. (Most
 plugins will only need a single completion contributor.)
 
 To write a completion contributor, create a class that implements
 `CompletionContributor`. The interface defines a single method named
 `computeSuggestions`. The method has two arguments: a `CompletionRequest` that
 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
 implement the method `computeSuggestions`. Your contributor should invoke the
 method `checkAborted`, defined on the `CompletionRequest` object, before
 starting any slow work. This allows the computation of completion suggestions
 to be preempted if the client no longer needs the results.
 
 For example, your contributor might look something like the following:
 
@@ skipping 14 matching lines @@
 class MyPlugin extends ServerPlugin with CompletionMixin, DartCompletionMixin {
   // ...
 
   @override
   List<CompletionContributor> getCompletionContributors(
       AnalysisDriverGeneric driver) {
     return <CompletionContributor>[new MyCompletionContributor()];
   }
 }
 ```