document accessor initargs and initform

Looks like this

    - [reader] <reader-name> <class-name> (<initargs> <= <initform>)

Reader for slot AAA of class FOO with initarg :AAA and initform NIL:

    - [reader] FOO-AAA FOO (:AAA <= NIL)

Writer for slot AAA of class FOO with initargs :AAA and :AAA-2 and no
initform:

    - [writer] FOO-AAA FOO (:AAA :AAA-2)

Accessor for slot AAA of class FOO with no initargs and initform NIL:

    - [accessor] FOO-AAA FOO (<= NIL)