(defvar refill-help-buffer-max-fill-column 68)

(defun refill-help-buffer (&optional frame)
  (interactive)
  (walk-windows
   #'(lambda (win)
       (with-selected-window win
         (save-excursion
           (when (and (eq major-mode 'help-mode)
                      (or (< fill-column refill-help-buffer-max-fill-column)
                          (window-truncated-p))
                      (progn
                        (goto-char (point-min))
                        (forward-sentence)
                        (re-search-backward "variable\\|function\\|command\\|face\\|form\\|macro" nil t)))
             (let (buffer-read-only)
               (setq fill-column (min refill-help-buffer-max-fill-column
                                      (- (window-width) 1)))
               (fill-region (point-min) (point-max)))))))
   'no-minibuffer frame))

(defun window-truncated-p (&optional window)
  "Return non-nil if current window (or the argument) has some
truncated lines."
  (with-selected-window (or window (selected-window))
    (save-excursion
      (goto-char (point-min))
      (re-search-forward
       (format "^..\\{%d\\}" (window-width)) nil t))))

(defun refill-help-buffer-reset-fill-column-and-refill nil
  (setq fill-column 0)
  (refill-help-buffer))

(define-minor-mode refill-help-buffer-mode
  "Automatically fill help buffer to
 \(min `refill-help-buffer-max-fill-column' \(window-width\)\)
 on displaying and resizing."
  nil nil nil
  :global t
  (if refill-help-buffer-mode
      (progn
        (add-hook 'window-size-change-functions 'refill-help-buffer)
        (add-hook 'temp-buffer-show-hook 'refill-help-buffer-reset-fill-column-and-refill))
    (remove-hook 'window-size-change-functions 'refill-help-buffer)
    (remove-hook 'temp-buffer-show-hook 'refill-help-buffer-reset-fill-column-and-refill)))

(provide 'refill-help-buffer)