* lisp/emacs-lisp/eieio*.el: Use (:documentation ...) feature

* lisp/emacs-lisp/eieio*.el: Use (:documentation ...) feature

* lisp/emacs-lisp/eieio-compat.el (eieio--defmethod): Use new
special (:documentation ...) feature.
* lisp/emacs-lisp/eieio-core.el (eieio-make-class-predicate)
(eieio-make-child-predicate): Same.
(eieio-copy-parents-into-subclass): Remove unused arg.
(eieio-defclass-internal): Adjust call accordingly and remove redundant
`pname' var.
(eieio--slot-name-index): Remove unused arg `obj' and adjust all
callers accordingly.