Add in-app documentation explaining how deployment end dates are calculated

[nathanielrindlaub]

Add to Deployment form. From docs:

NOTE: the End Dates for Deployments are determined automatically. For example, if Deployment B starts after Deployment A, the End Date for Deployment A is the Start Date for Deployment B. If Deployment A is the most recent, an End Date will not be set as it’s considered a current and on-going Deployment.