Treat list-buffers-directory as a string

Treat list-buffers-directory as a string

Another step in the long history of list-buffers-directory.  A thread
branch discussing the meaning/use of the variable starts here
https://lists.gnu.org/archive/html/emacs-devel/2009-09/msg00684.html
Also see (info "(elisp) Buffer File Name").
* lisp/buff-menu.el: Relocate special case code into info.el.  Nix
Info-* defvars.
(Buffer-menu--pretty-file-name): Remove special case.  Use
bound-and-true-p.
(Buffer-menu-info-node-description): Remove.
* lisp/ibuffer.el (ibuffer-buffer-file-name): Treat
list-buffers-directory as a string.
* lisp/info.el (Info-node-description): New function.
(Info-select-node): Use it.