bug#62279: Add an option to restore C-h b "Prefix Command" output

[Eli Zaretskii <eliz@gnu.org>]

> From: Bob Rogers <rogers@rgrjr.com>
> Date: Sun, 19 Mar 2023 12:26:46 -0700
> 
>    Pursuant to the 'No more "Prefix Command" in C-h m' discussion in the
> emacs-devel list.

Thanks.

> --- a/etc/NEWS
> +++ b/etc/NEWS
> @@ -61,6 +61,11 @@ This allows the user to customize the prompt that is appended by
>  This is used for displaying the time and date components of
>  'display-time-mode'.
>  
> +** New user option 'describe-bindings-show-prefixes'.
> +If true (the default is false), restores the "Prefix Command" entries

We use "If non-nil", not "If true".  And similarly for "false".

> +in 'describe-binding' ('C-h b') output for keys that are prefixes, the
> +details of which are shown later (regardless of this option setting).

This NEWS entry should go to where the change in "C-h b"s behavior is
described in NEWS (on emacs-29 branch), and should be reworded to tell
users how to get the previous behavior back ("To get the previous
behavior back, customize ...").

> +(defcustom describe-bindings-show-prefixes nil
> +  "Non-nil shows prefixes in the output buffer of `describe-bindings'."
> +  :type 'boolean
> +  :group 'help
> +  :version "30.1")

This should be "29.1".  Since the behavior change was introduced in
Emacs 29, we need to add this option in the same version.