Where is USE_FEATURES documented?

I replied to a PR about including libgetopt buildlink3.mk.

But when I looked at another example I saw it was done with USE_FEATURES.

I don't see any reference to libgetopt under mk/features. I didn't check, 
but I guess it uses nbcompat.

Where are the instructions?

The PR is 37253. (Another related is 33012 which I guess can be closed.)



  Jeremy C. Reed