bug#57937: 29.0.50; Improve doc of (interactive current-prefix-arg ...)

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Kangas <stefankangas@gmail.com>
> Date: Mon, 19 Sep 2022 12:12:52 -0400
> 
> Severity: wishlist
> 
> I think the explanation in (info "(elisp) Using Interactive") could
> include a pointer to `current-prefix-arg', which AFAICT is only
> documented much later in (info "(elisp) Prefix Command Arguments").

There is already such a pointer there:

   There are three possibilities for the argument ARG-DESCRIPTOR:

   • It may be omitted or ‘nil’; then the command is called with no
     arguments.  This leads quickly to an error if the command requires
     one or more arguments.

   • It may be a string; its contents are a sequence of elements
     separated by newlines, one for each argument(1).  Each element
     consists of a code character (*note Interactive Codes::) optionally
     followed by a prompt (which some code characters use and some
     ignore).  Here is an example:

          (interactive "P\nbFrobnicate buffer: ")

     The code letter ‘P’ sets the command’s first argument to the raw
     command prefix (*note Prefix Command Arguments::).