Provide documentation for SystemSettings.GuiConfigData.

[kubark42]

It's a UAVO field that seems to encode various vehicle frame actuator names into bitfields. It's memory efficient, since it stores the entire model setup into just 4 bytes, but it's fragile and needs to be well documented in order to prevent errors when extending models.

Alternatively, now that memory is no longer a consideration, we could consider spending a few more bytes and have category specific UAVOs. Since only one airframe UAVO would be loaded into memory at a time, the ram costs would be low.