vim-patch:8.1.0225: mode() does not indicate using CTRL-O from Insert mode (#9644)

Problem:    Mode() does not indicate using CTRL-O from Insert mode.
Solution:   Add "niI", "niR" and "niV" to mode() result. (closes vim/vim#3000)
612cc3888b

runtime/doc/eval.txt

@@ -5774,6 +5774,9 @@ mode([expr])	Return a string that indicates the current mode.
 		This is useful in the 'statusline' option or when used
 		with |remote_expr()| In most other places it always returns
 		"c" or "n".
+		Note that in the future more modes and more specific modes may
+		be added. It's better not to compare the whole string but only
+		the leading character(s).
 		Also see |visualmode()|.
 
 msgpackdump({list})					*msgpackdump()*