enable documentaiton keyword for defclass

In https://github.com/euslisp/EusLisp/blob/master/lisp/l/common.l#L742, there are :documentation keyword for make-class function and defclass is defined as macro in https://github.com/euslisp/EusLisp/blob/master/lisp/l/common.l#L821 .

but 
```
-			      documentation
-			      (doc documentation))
```
is something strange.

c.f.
documentation of class is supported in https://github.com/euslisp/EusLisp/blob/master/lisp/l/packsym.l#L78