fix(messages): no message kind for :undo messages #31590

Problem: cannot handle `:undo` and `:redo` messages in a special way,
e.g. replace one by another.

Solution: add `undo` kind.

runtime/doc/ui.txt

@@ -805,6 +805,7 @@ must handle.
 		"quickfix"	Quickfix navigation message
 		"search_cmd"	Entered search command
 		"search_count"	Search count message ("S" flag of 'shortmess')
+		"undo"		|:undo| and |:redo| message
 		"wildlist"	'wildmode' "list" message
 		"wmsg"		Warning ("search hit BOTTOM", |W10|, …)
 	    New kinds may be added in the future; clients should treat unknown