The :initarg for the documentation of a method is :documentation

src/clos/kernel.lsp

@@ -162,7 +162,7 @@
       (specializers :initarg :specializers :accessor method-specializers)
       (qualifiers :initform nil :initarg :qualifiers :accessor method-qualifiers)
       (function :initarg :function :accessor method-function)
-      (documentation :initform nil :initarg documentation)
+      (documentation :initarg :documentation :initform nil)
       (plist :initform nil :initarg :plist :accessor method-plist)
       (keywords :initform nil :accessor method-keywords))))
 

src/clos/method.lsp

@@ -62,7 +62,7 @@
 	  (multiple-value-bind (fn-form options)
 	      (make-method-lambda generic-function method lambda-form env)
 	    (when documentation
-	      (setf options (list* 'documentation documentation options)))
+	      (setf options (list* :documentation documentation options)))
 	    (multiple-value-bind (wrapped-lambda wrapped-p)
 		(simplify-lambda name fn-form)
 	      (unless wrapped-p