Document the `request.app` property.

* Mention it in the Request documentation
* Mention it in the Application documentation in the context of
  accessing the application state

docs/applications.md

@@ -80,7 +80,7 @@ exceptions that occur within the application:
 ### Storing state on the app instance
 
 You can store arbitrary extra state on the application instance, using the
-generic `app.state` attribute.
+generic `app.state` attribute.  This can then be accessed in middleware or endpoints via `request.app`.
 
 For example:
 

docs/requests.md

@@ -140,6 +140,10 @@ filename = form["upload_file"].filename
 contents = await form["upload_file"].read()
 ```
 
+#### Application
+
+The originating Starlette application can be accessed via `request.app`.
+
 #### Other state
 
 If you want to store additional information on the request you can do so