Need to update the conceptual docs for pub/sub for declarative subscribe

[msfussell]

https://github.com/dapr/docs/pull/784/files needs to also have the conceptual docs update https://github.com/dapr/docs/tree/master/concepts/publish-subscribe-messaging for this feature

[msfussell]

This feature is not in the docs and therefore unknown.

[yaron2]

This feature is not in the docs and therefore unknown.

Not correct, it's listed here: https://docs.dapr.io/developing-applications/building-blocks/pubsub/howto-publish-subscribe/#declarative-subscriptions