Re: doc elisp intro cross reference fixes

[Luc Teirlinck]

>From my previous message:

   ELISP> (get nil 'custom-group)
   ((ada-tight-gvd-integration custom-variable)
    (nnimap-authinfo-file custom-variable)
    (nnimap-prune-cache custom-variable)
    (nntp-authinfo-file custom-variable))

Actually, this only shows up after one does 

`M-x customize-group RET nil RET'

or similar

In the ielm run below, we have an emacs -q started from scratch.
We evaluate (symbol-plist 'nil) and get nil.  We do 
`M-x customize-browse' and evaluate again.  Then we do
`M-x customize-group RET nil RET' and evaluate a third time:

ELISP> (symbol-plist 'nil)
nil
ELISP> (symbol-plist 'nil)
(custom-loads
 (ada-xref nnimap nntp))

ELISP> (symbol-plist 'nil)
(custom-loads
 (ada-xref nnimap nntp)
 custom-group
 ((ada-tight-gvd-integration custom-variable)
  (nnimap-authinfo-file custom-variable)
  (nnimap-prune-cache custom-variable)
  (nntp-authinfo-file custom-variable)))

ELISP>