Add request.user to example (#905)

Newcomers get very confused if `request.user` is not set. I think it makes sense to underline this in the example.

docs/helpers.rst

@@ -130,8 +130,13 @@ Example
 
     from myapp.views import my_view
 
-    def test_details(rf):
+    def test_details(rf, admin):
         request = rf.get('/customer/details')
+        # Remember that when using RequestFactory, the request does not pass
+        # through middleware. If your view expects fields such as request.user
+        # to be set, you need to set them explicitly.
+        # The following line sets request.user to an admin user.
+        request.user = admin
         response = my_view(request)
         assert response.status_code == 200