cluster-discovery: rewrite guide

Am I too hung up on explaining how this works?

Is there enough info about cloud-config?

cc @philips
The cluster represented by the discovery URL is not started, and machine1 set '_state' to be 'started', and bootstrap a 1-node cluster.

I want to make sure that people won't have the doubt that why a discovery URL that contains no active machines, which have been deleted due to TTL expiration, cannot be reused.