bug#67542: Warn of shell-command-default-error-buffer non-notification

[Dan Jacobson]

On (info "(emacs) Single Shell") Please change:

By default, error output is intermixed with the regular output in the
output buffer.  But if you change the value of the variable
‘shell-command-default-error-buffer’ to a string, error output is
< inserted into a buffer of that name.
> silently inserted into a buffer of that name. So be sure to check it for any.