docs: document the nil case for dirvish-default-layout.

[nicolas-graves]

Took me quite some time to found how I could disable the additional windows when starting (see #197). If you do not intend to change the code, I think this could be useful to more than me and should be documented. Hence this pull request.

[nicolas-graves]

Actually, that wasn't enough. In the end, I created a lightly reworked version of dirvish-layout-toggle for what I wanted :

(setq dirvish-default-layout nil)
(defvar rde-dirvish-default-layout '(1 0.11 0.55)
 "The default dirvish layout when not in fullscreen.")

(defun rde-dirvish-layout-toggle ()
  "Toggle layout of current Dirvish session.
A session with layout means it has a companion preview window and
possibly one or more parent windows."
  (interactive)
  (let* ((dv (dirvish-curr))
          (old-layout (dv-layout dv))
          (new-layout (unless old-layout
                           (or (dv-last-fs-layout dv) rde-dirvish-default-layout)))
          (buf (current-buffer)))
    (if old-layout
        (set-window-configuration (dv-winconf dv))
        (with-selected-window (dv-root-window dv) (quit-window)))
     (setf (dv-layout dv) new-layout)
     (with-selected-window (dirvish--create-root-window dv)
                                (dirvish-save-dedication (switch-to-buffer buf))
                                (dirvish--build dv)
                                (dirvish-debounce nil (dirvish-preview-update dv)))))