Improve private option doc in CLI command

[gberl002]

When creating an edge router (ziti create config router edge ...) there is a flag for creating a private router (--private) but the description doesn't give any information on what a private router is, just simply states "Create a private router config"

Should add some clarification to explain that this is an edge router that is accessible from the network but is not broadcasting its existence... or however to best explain it.