Re: doc elisp intro cross reference fixes

[Richard Stallman]

        ** defcustom and other custom declarations now use a default group
        (the last group defined in the same file) when no :group was given.

There are two separate issues here:

* That the group can be specified by default when it isn't specified
  explicitly.

* What to do when there's a defcustoms for which no group is specified,
  neither explicitly nor by default.

We started this thread with the latter issue.  Let's not mix them up.

I think such a defcustom should cause a warning that reminds the
programmer to specify a group.