fix(server/usage): fixup the usage based on feedback

People were getting confused by the lack of explanation on which port to
use. Fix this.

/cc @polvi

server/usage.go

@@ -26,8 +26,9 @@ Options:
   -vv               Enabled very verbose logging.
 
 Cluster Configuration Options:
-  -peers=<peers>      Comma-separated list of peers (ip + port) in the cluster.
-  -peers-file=<path>  Path to a file containing the peer list.
+  -peers-file=<path>              Path to a file containing the peer list.
+  -peers=<host:port>,<host:port>  Comma-separated list of peers. The members
+                                  should match the peer's '-peer-addr' flag.
 
 Client Communication Options:
   -addr=<host:port>   The public host:port used for client communication.