Minor improvements to EDE and EIEIO manuals

Minor improvements to EDE and EIEIO manuals

For discussion, see the following threads:
https://lists.gnu.org/archive/html/emacs-devel/2020-05/msg00630.html
https://lists.gnu.org/archive/html/emacs-devel/2020-06/msg00099.html

* doc/misc/ede.texi (ede-generic-project): Clean up example.
* doc/misc/eieio.texi (Accessing Slots): Document slot-value as a
generalized variable and set-slot-value as obsolete.
(Predicates): Fix typo.
(Introspection): Document eieio-class-slots in place of the obsolete
object-slots.