Improve the rollbacks Runbook to include steps to unblock the pipeline following an uncompleted deployment

As discovered by @nolith, rollbacks will be blocked if a deployment is stopped prior to the finish stage and we decide to rollback. We are blocked because we check our locking mechanism prior to any deploys. This is desired to force us to check for on-going deployments prior to proceeding forward or backward. We must leave the locking mechanism in place, but we now need to update our documentation to ensure our runbook helps us account for this.

Alessio already documented the steps that we need to complete here: production#4206 (comment 561449456)

Let's utilize this issue to update our runbook appropriately.