bug#20992: 25.0.50; `virtual-dired': (1) doc typo, (2) unclear

[Phil Sainty]

Well I now see that `dired-virtual-mode' actually suggests some
config, which I'd managed to not notice:


> Useful on `magic-mode-alist' with the regexp
>
>   "^  \\(/[^ /]+\\)+/?:$"
>
> to put saved Dired buffers automatically into Virtual Dired mode.
>
> Also useful for `auto-mode-alist' like this:
>
>   (add-to-list 'auto-mode-alist
>                '("[^/]\\.dired\\'" . dired-virtual-mode))


I note that the suggested regexp doesn't support directory paths
containing spaces.

I'd opted to match more of the output than that, as it seemed a bit
brittle to assume a dired buffer based only on the first line.