Re: doc elisp intro cross reference fixes

[Luc Teirlinck]

>From a prior message:

   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

Of course, (user-variable-p 'a) returns t, even in 21.3 and 
M-x set-variable RET a RET worked, even in 21.3 (Messed up my quotes
again).  Nevertheless the other claims in that message were valid.

Sincerely,

Luc.