bug#39138: 28.0.50; Add variables for shell command output buffer names

[Juri Linkov <juri@linkov.net>]

>> Oh, I noticed that the patch provided by Tino removed all mentions of
>> shell buffer names from the documentation.  While technically this is fine,
>> I tried to look from the point of view of users, and realized that it's
>> highly unlikely that the users might be interested to know the name of
>> the constant 'shell-command-buffer-name', whereas it's very important
>> for users to learn that the default name of the shell output buffer is
>> "*Shell Command Output*".
>
> I don't really see why?  If the user is using one of these commands, of
> course they'll see the buffer names themselves.

Often they don't see the buffer names - when the shell output is less
than 16 lines long, then the shell buffer is not displayed, so users
need to know the shell buffer name to be able to switch to it.  They need
to learn the buffer name from the User Manual.

> If they're writing code, they should be using 'shell-command-buffer-name',
> and not the string "*Shell Command Output*".

I completely agree that Emacs Lisp programmers need to know the new way
of using the shell buffer name via the new constant to reduce typos
in buffer names.  So the new constant should be explained in the
Emacs Lisp Reference Manual.

But the problem is that these constants are currently described
in the User Manual that make no sense to end users - users need
to know the buffer names, not internal constant names.