connection: Document rawConn

Co-Authored-By: vmg <vicent@github.com>

Author: methane

connection.go

@@ -22,7 +22,7 @@ import (
 type mysqlConn struct {
 	buf              buffer
 	netConn          net.Conn
-	rawConn          net.Conn
+	rawConn          net.Conn  // underlying connection when netConn is TLS connection.
 	affectedRows     uint64
 	insertId         uint64
 	cfg              *Config