[doc] describe the way fabric:container-delete works for containers "created" by fabric:join

We should document the way container-delete works when we have not complete metadata about containers (the metadata is only present where containers where created by e.g. fabric:container-create-child) created by fabric:join.

I incorporated fixes into the 6.1 patch branch which actually allow to delete such containers by cleaning up ZK registry. So for these containers (fabric:join-ed), we just clean the registry. User should be aware that the containers might still exist and might eventually reconnect to the ensemble and register themselves again. That's why I'm working on fabric:leave command.

See ENTESB-1759, ENTESB-1764, ENTESB-1773, ...