bug#73505: EIEIO manual out of date, doesn't reflect current functionality

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> But this isn't true at all, EIEIO has both of these features fully
> functional in fact.

It's both true and false:

- EIEIO "has" none of those things because the defmethod stuff is now
  provided by something else than EIEIO.  EIEIO only provides `defclass`
  and associated functions (e.g. `slot-value`).

- But yes, the text is out of date because you can now use these
  features (with EIEIO objects, with defstruct objects, with
  oclosure objects, ...).

> The EIEIO manual states that it provides method functionality for
> Emacs Lisp.  The manual has whole sections devoted to both
> cl-defmethod and cl-defgeneric and refers to them as part of its
> corpus.  It has documentation for both.
>
>     (info "(eieio) Generics")
>     (info "(eieio) Methods")

This structure is a left-over from when generics were provided by EIEIO.
We should clarify.

> And as far as I can tell there is no eieio-specific defmethod or
> defgeneric macros that aren't cl-defmethod and cl-defgeneric.
> So I really doubt that I'm in error here.

Actually, `eieio-compat` still provides the old `defmethod` and
`defgeneric` (without `cl-`)


        Stefan