Update HA Documentation on enabling a replica

We need to update the documentation at https://puppet.com/docs/pe/2018.1/configure_high_availability.html#enable-a-replica 

specifically, this point:

4. On the replica, in the /etc/puppetlabs/puppet/puppet.conf file's main section, remove the entire line where the server variable is set: server = <MASTER NODE NAME>

When you enable HA, the server variable is superseded by the server_list variable. If you later promote a replica on which the server variable still exists and points to the old master, some commands might not function properly.

That should be updated to more clearly define the interaction between server and server_list when you set one, both, or none in /etc/puppetlabs/puppet/puppet.conf.

If server_list is set, we always take the first value in that array. Otherwise, if server_list is not set, we use server.