Add comment documentation everywhere

[stylianosnicoletti]

Follow this approach:

 /**
 * Adds a tear down to be called during the unsubscribe() of this
 * 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
 * `Subscription.EMPTY`, it will not be added.
 *
 * 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
 * added to the inner subscriptions list. This Subscription can be used with
 * `remove()` to remove the passed teardown logic from the inner subscriptions
 * list.
 */