doc(Decorate): Explain that decorators can not add new values to the …

…graph

I created uber-go#1144 because I was not sure whether a decorator
could add new values to the graph.

This clarifies the behavior.
We already have tests to verify this behavior.

Resolves uber-go#1144

decorate.go

@@ -123,6 +123,11 @@ import (
 //	  return r
 //	}),
 //
+// Decorators can not add new values to the graph,
+// only modify or replace existing ones.
+// Types returned by a decorator that are not already in the graph
+// will be ignored.
+//
 // # Decorator scope
 //
 // Modifications made to the Fx graph with fx.Decorate are scoped to the