[doc]: specify node visibility requirements in the doc

[dberardo-com]

i could not find out fro the documentation alone, whether in an HA setup:

• all server nodes must be able to see each others
• all agent nodes must be able to see ALL other server and/or agent nodes

in particular in the second part: is it possible that one agent node only sees on server node ? and of course if that server node goes down, then the agent goes down with it, sure.

is it possible to achieve this? and in such case is ti possible to specify a list of preferred server nodes in the agent config file so that when it starts it does not go over all server ips waiting for the first one to respond ? this makes the reboot very long