Re: doc elisp intro cross reference fixes

[Luc Teirlinck]

Per Abrahamsen wrote:

   Maybe it is because I use an older Emacs.

This `nil' group feature does not exist in emacs-21.3.

In emacs-21.3, (defcustom a 4 "c") will give `a' value 4, but `a' will
not get a custom group and `customize-variable RET a' will say:

"Variable a cannot be customized"

(user-variable-p a) in emacs-21.3 returns nil

Sincerely,

Luc.