Corrected status of subtyping.md

Change-Id: Iaede132c879a63bd2722593b284c8a791971190f
Reviewed-on: https://dart-review.googlesource.com/c/87064
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
diff --git a/docs/language/informal/subtyping.md b/docs/language/informal/subtyping.md
index 64c0f29..65df0ea 100644
--- a/docs/language/informal/subtyping.md
+++ b/docs/language/informal/subtyping.md
@@ -2,7 +2,8 @@
 
 leafp@google.com
 
-**Status**: Implemented.
+**Status**: This document is now background material.
+For normative text, please consult the language specification.
 
 This is intended to define the core of the Dart 2.0 static and runtime subtyping
 relation.