Update Vehicle::guidedModeGotoLocation doc comment

[Maarrk]

Both ArduPilot firmware and PX4 discard the altitude when called by this function:

https://github.com/mavlink/qgroundcontrol/blob/16f59b49208fe840ab511b5726a8e506c18681e7/src/FirmwarePlugin/APM/APMFirmwarePlugin.cc#L848

https://github.com/mavlink/qgroundcontrol/blob/16f59b49208fe840ab511b5726a8e506c18681e7/src/FirmwarePlugin/PX4/PX4FirmwarePlugin.cc#L487

I created the PR with the most obvious and least invasive fix, because for end-user there are still possibilities to change the altitude. If there would be a need to implement the API to call it with altitude, I'd contribute it guided by a maintainer what would be most useful for you