bug#56863: 28.1; "internal variable" in doc

[Eli Zaretskii]

> From: Drew Adams <drew.adams@oracle.com>
> Date: Sun, 31 Jul 2022 20:17:58 +0000
> 
>  (defvar foo 50)
> (defvaralias 'fill-column 'foo)
> 
> --> Error "Cannot make an internal variable an alias"
> 
> The error msg should show the name of the variable in question.

I added the name to the message.

> But the real bug is that "internal variable" isn't clear/defined.

I made it say "built-in" instead of "internal".

Thanks.