Documenting app_name CLI argument

Adding documentation of when the app_name argument is needed in the command line, and made it more clear that each command only operates on a single application.

Readers of the old documentation could mistakenly assume that the commands will apply to all bluepill applications if no app_name argument was provided. For example running "status" without specifying an app_name will show the status of all bluepill applications.

Or worse, they test their scripts with no app_name specified and it works, since they are only testing their own bluepill application. But in deployment, an unrelated application also uses bluepill and suddenly their scripts fail.