Add ISO number for longitudinal limits flag comment

board/safety_declarations.h

@@ -101,6 +101,7 @@ bool brake_pressed_prev = false;
 
 // If using this flag, be aware that harder braking is more likely to lead to rear endings,
 //   and that alone this flag doesn't make braking compliant because there's also a time element.
+// See ISO 15622:2018 for more information.
 #define UNSAFE_RAISE_LONGITUDINAL_LIMITS_TO_ISO_MAX 8
 
 int unsafe_mode = 0;