Update interop-test-descriptions.md

doc/interop-test-descriptions.md

@@ -750,9 +750,29 @@ Client asserts:
 
 ### unimplemented_method
 
-Status: Ready for implementation. Blocking beta.
+This test verifies that calling an unimplemented RPC method returns the 
+UNIMPLEMENTED status code
 
-This test verifies calling unimplemented RPC method returns the UNIMPLEMENTED status code.
+Server features:
+N/A
+
+Procedure:
+* Client calls `grpc.testing.TestService/UnimplementedMethod` with an empty
+  request (defined as `grpc.testing.Empty`):
+
+    ```
+    {
+    }
+    ```
+   
+Client asserts:
+* received status code is 12 (UNIMPLEMENTED)
+* received status message is empty or null/unset
+
+### unimplemented_service
+
+This test verifies calling an unimplemented server returns the UNIMPLEMENTED
+status code.
 
 Server features:
 N/A