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

closes #1960

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

@@ -430,11 +430,20 @@ block injectable-not-always-needed-in-ts
 +ifDocsFor('ts')
   .l-sub-section
     :marked
-      Injectors use a class's constructor metadata to determine dependent types as
+      At runtime, injectors can read class metadata in the transpiled JavaScript code
-      identified by the constructor's parameter types.
+      and use the constructor parameter type information
-      TypeScript generates such metadata for any class with a decorator, and any decorator will do.
+      to determine what things to inject. 
-      But of course, it is more meaningful to mark a class using the appropriate
+
-      <a href="#{injMetaUrl}">InjectableMetadata</a> #{_decorator}.
+      Not every JavaScript class has metadata.
+      The TypeScript compiler discards metadata by default.
+      If the `emitDecoratorMetadata` compiler option is true 
+      (as it should be in the `tsconfig.json`),
+      the compiler adds the metadata to the generated JavaScript 
+      for _every class with at least one decorator_.
+
+      While any decorator will trigger this effect, mark the service class with the
+      <a href="#{injMetaUrl}">InjectableMetadata</a> #{_decorator}
+      to make the intent clear.
 
 .callout.is-critical
   header Always include the parentheses