feat(messages): confirm kind for z=, :tselect, inputlist() #32521

Problem:  Messages preceding a `cmdline_show->prompt` event can not be
          distinguished as such when receiving the event. (But since
          `msg_show` handlers should be scheduled, one can already check
          whether a prompt is active when displaying the message.)

Solution: Rather than add a new kind again, use the `confirm` kind.
          Could be seen as slightly misleading where it is more of
          a choice rather than a confirmation, but that already applies
          to `confirm()` as well...

runtime/doc/ui.txt

@@ -791,7 +791,8 @@ must handle.
 	    Name indicating the message kind:
 		"" (empty)	Unknown (consider a |feature-request|)
 		"bufwrite"	|:write| message
-		"confirm"	|confirm()| or |:confirm| dialog
+		"confirm"	Message preceding a prompt (|:confirm|,
+				|confirm()|, |inputlist()|, |z=,|, …)
 		"emsg"		Error (|errors|, internal error, |:throw|, …)
 		"echo"		|:echo| message
 		"echomsg"	|:echomsg| message