diff --git a/lisp/simple.el b/lisp/simple.el
index d4ae5ebb1f..0274bdb459 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -8188,6 +8188,9 @@ choose-completion-string-functions
 BUFFER - the buffer in which the choice should be inserted,
 BASE-POSITION - where to insert the completion.
 
+Functions should also accept and ignore a potential fourth
+argument, passed for backwards compatibility.
+
 If a function in the list returns non-nil, that function is supposed
 to have inserted the CHOICE in the BUFFER, and possibly exited
 the minibuffer; no further functions will be called.