Document the deprecation of hardcoded cave liquids (#8692)

src/mapgen/mapgen.h

@@ -278,5 +278,7 @@ protected:
 	float cavern_limit;
 	float cavern_taper;
 	float cavern_threshold;
+	// TODO 'lava_depth' is deprecated and should be removed. Cave liquids are
+	// now defined and located using biome definitions.
 	int lava_depth;
 };