Document "wielditem" visual

doc/lua_api.txt

@@ -2010,7 +2010,7 @@ Object Properties
     collide_with_objects = true, -- collide with other objects if physical=true
     weight = 5,
     collisionbox = {-0.5,-0.5,-0.5, 0.5,0.5,0.5},
-    visual = "cube"/"sprite"/"upright_sprite"/"mesh",
+    visual = "cube"/"sprite"/"upright_sprite"/"mesh"/"wielditem",
     visual_size = {x=1, y=1},
     mesh = "model",
     textures = {}, -- number of required textures depends on visual