Improve some doc strings in bookmark.el

Improve some doc strings in bookmark.el

* lisp/bookmark.el (bookmark-bmenu-load): Describe prefix argument
  behavior.  Refer to related functions for more information.
  (bookmark-bmenu-save): Likewise refer to related functions.

As discussed in this thread:

  https://lists.gnu.org/archive/html/emacs-devel/2021-05/msg00389.html

  From: Karl Fogel
  To: Eli Zaretskii
  Cc: Matthias Meulien, Drew Adams, Lars Ingebrigtsen,
      Stefan Monnier, Emacs Devel
  Subject: Re: [External] : Re: [PATCH] When deleting in bookmark menu,
           prompt for confirmation.
  Date: Sun, 09 May 2021 13:37:52 -0500
  Message-ID: <87h7jboirj.fsf@red-bean.com>