bug#30503: 27.0.50; allow hiding M-x grep command line

[Juri Linkov]

>> > In any case, this needs to be documented in NEWS and the manual, I
>> > think.
>>
>> Changes to NEWS are added in this patch without “---” to not forget
>> to document it in the manual before the release of Emacs 27.1.
>
> Thanks, but I'd prefer that we update the manual as part of this
> changeset.  Leaving manuals outdated tends to create a very large job
> to be done before a pretest can be released, which I find to be
> sub-optimal.  It also prevents users who track the development from
> finding possible mistakes in the manual, because they don't have
> enough time to d that before an official release.
>
> Would you like to make the corrections in the manual as well?

Pushed to master together with the corrections in the manual.

Charles, please update your patch from master, you have many
good improvements in your previous patch.