bug#70212: Customize Group buffer: add button to reveal underlying variable names

[Juri Linkov]

> The user is in buffer
> *Customize Group: Compilation*
>
> The user sees:
> Show Value Compilation Max Output Line Length
>    Output lines that are longer than this value will be hidden. Hide
>    If nil, don’t hide anything.
>
> Try as the user might, there is no way for him to discover the
> underlying variable there in the interface.

You can read the related discussion at bug#41905.