message: docs: clarify providedIn options
  The list for the possible options of providedIn was not totally clear. This commit ensures each possible value is included explicitly in the docs.
  fixes
  PR Close
diff: @@ -64,9 +64,15 @@ export interface InjectableDecorator {
  */
  export interface Injectable {
  /**
  - * Determines which injectors will provide the injectable,
  - * by either associating it with an `@NgModule` or other `InjectorType`,
  - * or by specifying that this injectable should be provided in one of the following injectors:
  + * Determines which injectors will provide the injectable.
  + *
  + * - `Type<any>` - associates the injectable with an `@NgModule` or other `InjectorType`,
  + * - 'null' : Equivalent to `undefined`. The injectable is not provided in any scope automatically
  + * and must be added to a `providers` array of an [@NgModule](api/core/NgModule#providers),
  + * [@Component](api/core/Directive#providers) or [@Directive](api/core/Directive#providers).
  + *
  + * The following options specify that this injectable should be provided in one of the following
  + * injectors:
  * - 'root' : The application-level injector in most apps.
  * - 'platform' : A special singleton platform injector shared by all
  * applications on the page.
  