document why servers and clients uses different port to join server 4647 VS 4648

[EugenKon]

1. This not clear information is documented on this page: https://developer.hashicorp.com/nomad/tutorials/manage-clusters/clustering#manual-clustering

2. Also please clarify 'The port corresponds to the RPC port. ' part. Which RPC port do you mean here: nomad, consul etc? Provide a link to corresponding configuration option.

3. This means only one server must be specified because, after initial contact, the full set of servers in the client's region are shared with the client.

   I do not remember where but I am sure that you used array of servers as the value for this option. Anyway we can see that you require array. Notice [] for the value. It is ambiguous is only one server must be specified why this one value is passed via array?

[tgross]

Looks like this detail is missing from the config docs as well: https://developer.hashicorp.com/nomad/docs/configuration/server_join#retry_join