Biomes: Document xyz biome limits

doc/lua_api.txt

@@ -5114,9 +5114,16 @@ Definition tables
         node_riverbed = "default:gravel",
         depth_riverbed = 2,
     --  ^ Node placed under river water and thickness of this layer.
-        y_min = 1,
         y_max = 31000,
-    --  ^ Lower and upper limits for biome.
+        y_min = 1,
+    --  ^ Upper and lower limits for biome.
+    --  ^ Alternatively you can use xyz limits as shown below.
+        max_pos = {x = 31000, y = 128, z = 31000},
+        min_pos = {x = -31000, y = 9, z = -31000},
+    --  ^ xyz limits for biome, an alternative to using 'y_min' and 'y_max'.
+    --  ^ Biome is limited to a cuboid defined by these positions.
+    --  ^ Any x, y or z field left undefined defaults to -31000 in 'min_pos' or
+    --  ^ 31000 in 'max_pos'.
         vertical_blend = 8,
     --  ^ Vertical distance in nodes above 'y_max' over which the biome will
     --  ^ blend with the biome above.