Callback: Extra information about layout in the build gui

From GRFSpecs

Extra information about airport layout in the build gui (155)

This callback allows you to display some extra text in the build airport gui. It should return a D0xx StringID.

GRFv≥8 For GRF version 8 and above you can return 400 to display no text (instead of failing the callback).

This callback is always called when available, you do not need to set a bit in any action0 property to enable it. Available for OpenTTD since r20272