Re: I don't understand this comment on message2

[Richard M. Stallman]

       The buffer M must continue to exist until after the echo area gets
       cleared or some other message gets displayed there.  This means do
       not pass text that is stored in a Lisp string; do not pass text in
       a buffer that was alloca'd.  */

I think that comment is probably obsolete.  Nowadays the echo area
text is stored in real buffers.  Before Emacs 20, I think it was
stored in null-terminated C strings.

You could double-check this by looking at the code from Emacs 19.
If this comment was needed for that reason, it can be deleted now.