patch 8.1.0716: get warning message when 'completefunc' returns nothing

Problem:    Get warning message when 'completefunc' returns nothing.
Solution:   Allow for returning v:none to suppress the warning message.
            (Yasuhiro Matsumoto, closes #3789)

runtime/doc/insert.txt

@@ -1076,6 +1076,10 @@ that contains the List.  The Dict can have these items:
 			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.