bug#48917: 28.0.50; allow user to choose what function is used when windmove create a window

[Eli Zaretskii <eliz@gnu.org>]

> From: pillule <pillule@riseup.net>
> Date: Tue, 08 Jun 2021 12:31:58 +0200
> 
>  (defcustom windmove-create-window nil
> -  "Whether movement off the edge of the frame creates a new window.
> +  "Whether movement off the edge of the frame creates a new window or
> +trigger a custom function.

The first line of a doc string should be a single complete sentence.
(The reason for this is that some Emacs commands, like "M-x apropos",
show only the first line of the doc string.)

>  If this variable is set to t, moving left from the leftmost window in
>  a frame will create a new window on the left, and similarly for the other
>  directions."
> -  :type 'boolean
> +  :type '(choice (const :tag "Don't create new windows" nil)
> +          (const :tag "Create new windows" t)
> +          (function :tag "Provide a function"))
>    :group 'windmove
>    :version "27.1")

The :version tag should be updated, because the defcustom was changed.