babel: adding comments to org-babel-header-arg-names

* lisp/ob.el (org-babel-header-arg-names): adding comments to
  org-babel-header-arg-names

lisp/ob.el

@@ -240,7 +240,7 @@ then run `org-babel-pop-to-session'."
 
 (defconst org-babel-header-arg-names
   '(cache cmdline colnames dir exports file noweb results
-	  session tangle var noeval)
+	  session tangle var noeval comments)
   "Common header arguments used by org-babel.  Note that
 individual languages may define their own language specific
 header arguments as well.")