docs(dependency-injection): Clarify how TypeScript generates decorator metadata

public/docs/ts/latest/guide/dependency-injection.jade

@@ -432,7 +432,8 @@ block injectable-not-always-needed-in-ts
     :marked
       Injectors use a class's constructor metadata to determine dependent types as
       identified by the constructor's parameter types.
-      TypeScript generates such metadata for any class with a decorator, and any decorator will do.
+      TypeScript generates such metadata for any class with a decorator (if 
+      `emitDecoratorMetadata` compiler option is set to true), and any decorator will do.
       But of course, it is more meaningful to mark a class using the appropriate
       <a href="#{injMetaUrl}">InjectableMetadata</a> #{_decorator}.