Implement sign using form field protocol

doc/lua_api.txt

@@ -1036,6 +1036,10 @@ Node definition (register_node)
 	on_dig = func(pos, node, digger),
     ^ default: minetest.node_dig
     ^ By default: checks privileges, wears out tool and removes node
+
+    on_receive_fields = func(pos, formname, fields, sender),
+    ^ fields = {name1 = value1, name2 = value2, ...}
+    ^ default: nil
 }
 
 Recipe: (register_craft)