diff --git a/runtime/doc/insert.txt b/runtime/doc/insert.txt
index a200fc5..e01eac9 100644
--- a/runtime/doc/insert.txt
+++ b/runtime/doc/insert.txt
@@ -1076,6 +1076,10 @@
 			The only value currently recognized is "always", the
 			effect is that the function is called whenever the
 			leading text is changed.
+
+If you want to suppress the warning message for an empty result, return
+v:none.  This is useful to implement asynchronous completion with complete().
+
 Other items are ignored.
 
 For acting upon end of completion, see the |CompleteDone| autocommand event.