document async nature of engine ops

[pferrel]

response to engine add, for instance, only means the "job" of adding has been accepted. It does not mean engine params are validated -- as of etcd version.

[pferrel]

the engine ops are now async with the REST calls so document this. For instance is hctl status engines does not return the correct params, then the logs must be searched for the engine op error, it is not retrievable via the API

[pferrel]

only applies to Harness 1.0+