SubscriberOptions are not documented anywhere

The [SubscriberOptions documentation](https://github.com/googleapis/nodejs-pubsub/blob/ac86600cda69e89ade8ae6ea60e4a21100ed89aa/src/subscriber.ts#L355) is seemingly not available anywhere other than the source code. We do not define them anywhere in the documentation we link to from our product documentation, though we refer to the type: https://googleapis.dev/nodejs/pubsub/latest/PubSub.html#subscription. We should audit and make sure all necessary types are defined in the documentation.