update the documentation for is_function()

doc/script_commands.txt

@@ -2040,16 +2040,19 @@ Example:
 
 ---------------------------------------
 
-*is_function("<function name>")
+*is_function("<function name>"{, <npc id>})
+*is_function("<function name>"{, "<npc name>"})
 
-This command checks whether or not a function exists and returns its type.
-Returns false if it cannot be found.
+This command checks whether or not a function exists and returns its type. If a
+NPC name or ID is provided, it will try to find the label or function within
+this NPC instead of the attached NPC. Returns false if it cannot be found.
 
 return values:
 
  FUNCTION_IS_COMMAND - built-in script command (eg: mes, select, ...)
  FUNCTION_IS_GLOBAL - user-defined global function (callable with callfunc)
  FUNCTION_IS_LOCAL - user-defined local function
+ FUNCTION_IS_PUBLIC - user-defined local function marked as public
  FUNCTION_IS_LABEL - user-defined label function (callable with callsub)
 
 Example: