bug#12309: Option `(bookmark-)version-control': Use :tag so doc string matches menu

[Karl Fogel <kfogel@red-bean.com>]

Fixed (see below), but please review.  

I don't fully understand the whole customization system because I never
use it myself (I just read doc strings and set variables directly in
Elisp), so I don't quite get what `other' mean if used instead of
`const', and I didn't fully understand the last paragraph of the
original bug report.  I looked in the Info pages, but they didn't
clarify much about this.

-Karl

Revision info:

  revno: 110305
  revision-id: kfogel@red-bean.com-20121001033206-5eja4ztyhs1sjm7q
  parent: cyd@gnu.org-20121001031702-2mei04wuzv2pk1e7
  committer: Karl Fogel <kfogel@red-bean.com>
  branch nick: trunk
  timestamp: Sun 2012-09-30 22:32:06 -0500
  message:
    * lisp/bookmark.el (bookmark-version-control): Give tags in the
      :type choices (Bug#12309), and improve doc string.

Diff:

  === modified file 'lisp/ChangeLog'
  --- lisp/ChangeLog	2012-10-01 02:07:14 +0000
  +++ lisp/ChangeLog	2012-10-01 03:31:41 +0000
  @@ -1,3 +1,8 @@
  +2012-10-01  Karl Fogel  <kfogel@red-bean.com>
  +
  +	* bookmark.el (bookmark-version-control): Give tags in the
  +	:type choices (Bug#12309), and improve doc string.
  +
   2012-10-01  Paul Eggert  <eggert@cs.ucla.edu>
   
   	Revert the FOLLOW-SYMLINKS change for file-attributes.
  
  === modified file 'lisp/bookmark.el'
  --- lisp/bookmark.el	2012-09-25 04:13:02 +0000
  +++ lisp/bookmark.el	2012-10-01 03:32:18 +0000
  @@ -99,12 +99,14 @@
   
   (defcustom bookmark-version-control 'nospecial
     "Whether or not to make numbered backups of the bookmark file.
  -It can have four values: t, nil, `never', and `nospecial'.
  +It can have four values: t, nil, `never', or `nospecial'.
   The first three have the same meaning that they do for the
  -variable `version-control', and the final value `nospecial' means just
  -use the value of `version-control'."
  -  :type '(choice (const nil) (const never) (const nospecial)
  -		 (other t))
  +variable `version-control'; the value `nospecial' (the default) means
  +just use the value of `version-control'."
  +  :type '(choice (const :tag "If existing" nil)
  +                 (const :tag "Never" never)
  +                 (const :tag "Use the value of `version-control'" nospecial)
  +                 (const :tag "Always" t))
     :group 'bookmark)