RGB LED description should mention parameter NTF_LED_OVERRIDE

[kd0aij]

The "standard" pattern is described here: https://ardupilot.org/copter/docs/common-leds-pixhawk.html#rgb-leds

but we should also mention that there are currently 2 other options: OBC and traffic light. Currently, the only description (that I could find) for those 2 options resides in the parameter metadata.

[Hwurzburg]

Feel free to add info to the page!

[kd0aij]

I would have done that, but I'm not sure there is quite enough info in the parameter description. So I'll defer the task to someone who knows more about it.