bug#40774: Error messages shouldn't be hidden when the user is idle

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: 40774@debbugs.gnu.org,  eliz@gnu.org,  ndame@protonmail.com
> Date: Sun, 05 Dec 2021 20:54:20 +0200
> 
> To be able to use clear-message-function in Emacs 29,
> we need to issue a notification in Emacs 28 NEWS:
> 
> diff --git a/etc/NEWS b/etc/NEWS
> index 8e38c3690c..69d00a8bf1 100644
> --- a/etc/NEWS
> +++ b/etc/NEWS
> @@ -3327,6 +3327,10 @@ This new 'etc-authors-mode' provides font-locking for 
> displaying the
>  
>  * Incompatible Lisp Changes in Emacs 28.1
>  
> +---
> +** The return value of 'clear-message-function' will change in later 
> releases.
> +When it will return a non-nil, the echo area won't be cleared.
> +

We never did anything like that before, AFAIR.  Why is this needed,
and how is this change different from any other change in behavior?