bug#38457: 27.0.50; dabbrev-expand regression due to message change

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: 38457@debbugs.gnu.org
> Date: Sun, 22 Dec 2019 00:02:48 +0200
> 
> Pushed with the nil default of minibuffer-message-clear-timeout.
> Also added documentation to docstrings and to the manual.
> Please check if everything is alright.

Thanks, it's fine (I made some minor wording changes).