gimbal_v2.md: Add clarification about gimbal yaw frame

[Davidsastresas]

Following the discussion here: mavlink/mavlink#2093 (comment)

@julianoes @hamishwillee let me know if it looks good to you. Thanks!

[hamishwillee]

Thanks very much @Davidsastresas . I've subedited to present this as "what you do first" and provide a little more context. Can you please check it still says what you want? @julianoes setting you as reviewer.

FYI also created mavlink/mavlink#2094 so there is some info in the message itself.

[Davidsastresas]

It is great, thank you @hamishwillee!

[rmackay9]

Sorry to be late to the party but I think we might want to clarify that for cases where the gimbal is providing the two new flags (GIMBAL_DEVICE_FLAGS_YAW_IN_VEHICLE_FRAME and GIMBAL_DEVICE_FLAGS_YAW_IN_EARTH_FRAME) we are still using the older GIMBAL_DEVICE_FLAGS_YAW_LOCK flag but now it is only used to hold the target (angle or rate) yaw frame.

So the new flags give the frame of the yaw angles and rates included in the GIMBAL_DEVICE_ATTITUDE_STATUS message. The GIMBAL_DEVICE_FLAGS_YAW_LOCK flag now says whether the GCS, user, whatever provided target yaw angles or rates in body-frame (aka follow) or earth-frame (aka lock).

[julianoes]

Correct @rmackay9

[hamishwillee]

Thanks @rmackay9 - Attempted to clarify in #532