message: docs(Subscription): add description for child subscription
  Make clear that add() can be used to add child subscriptions.
diff: @@ -124,7 +124,7 @@ export class Subscription implements SubscriptionLike {
  /**
  * Adds a tear down to be called during the unsubscribe() of this
  - * Subscription.
  + * Subscription. Can also be used to add a child subscription.
  *
  * If the tear down being added is a subscription that is already
  * unsubscribed, is the same reference `add` is being called on, or is
  @@ -133,6 +133,8 @@ export class Subscription implements SubscriptionLike {
  * If this subscription is already in an `closed` state, the passed
  * tear down logic will be executed immediately.
  *
  + * When a parent subscription is unsubscribed, any child subscriptions that were added to it are also unsubscribed.
  + *
  * @param {TeardownLogic} teardown The additional logic to execute on
  * teardown.
  * @return {Subscription} Returns the Subscription used or created to be
  