Add comment to eieio-opt about why we're requiring cl-extra

* lisp/emacs-lisp/eieio-opt.el (cl-extra): Add comment about why
we're not requiring cl-lib.

lisp/emacs-lisp/eieio-opt.el

@@ -31,6 +31,9 @@
 (require 'eieio)
 (require 'find-func)
 (require 'speedbar)
+
+;; We require cl-extra here instead of cl-lib because we need the
+;; internal `cl--describe-class' function.
 (require 'cl-extra)
 
 ;;; Code: