bug#39722: Support for bookmark.el in VC directory buffers

[Juri Linkov]

> Being able to bookmark VC directory buffers is useful when working on
> multiple projects; The proposed patch implements this on the lines of
> what I read in info.el.

Thanks, this is a useful addition.  Please push it after fixing minor
details:

> +(defun vc-dir-bookmark-make-record ()
> +  "This implements the `bookmark-make-record-function' type for
> +`vc-dir' buffers."
> ...
> +(defun vc-dir-bookmark-jump (bmk)
> +  "This implements the `handler' function interface for the record
> +type returned by `vc-dir-bookmark-make-record'."

According to the documentation standard described in
(info "(elisp) Documentation Tips")
the first line of the docstring should consist of a complete sentence.

I see that you copied the docstring from Info-bookmark-jump,
but the docstrings of info.el contains a mistake
that should be fixed in info.el.