doc(server): some basic docs on the tls_config object

This should be refactored but something to remember while refactoring.

server/tls_config.go

@@ -6,7 +6,7 @@ import (
 
 // TLSConfig holds the TLS configuration.
 type TLSConfig struct {
-	Scheme string
-	Server tls.Config
-	Client tls.Config
+	Scheme string     // http or https
+	Server tls.Config // Used by the Raft or etcd Server transporter.
+	Client tls.Config // Used by the Raft peer client.
 }