documentation slot option

[Daniel Cerys <cerys@bbn.com>]

In DEFCLASS, use of the the :documentation slot option results in an
error.  Even if the :documentation value isn't used by the system, it
shouldn't result in an error.

The following change to defclass.lisp corrects this problem:  

(defmethod LEGAL-SLOT-OPTION-P ((class standard-class) option)
  (memq option '(:name :initform :initfunction :initarg :initvalue :type
:accessor :reader :allocation
		 :documentation)))

[Note that :writer is also not supported, but unlike :documentation,
its value cannot simply be ignored.]