checkdoc-spellcheck-documentation-flag doc string improvement

* lisp/emacs-lisp/checkdoc.el
(checkdoc-spellcheck-documentation-flag): Mention
`ispell-kill-ispell' (bug#6221).

lisp/emacs-lisp/checkdoc.el

@@ -241,7 +241,12 @@ system.  Possible values are:
   defun       - Spell-check when style checking a single defun.
   buffer      - Spell-check when style checking the whole buffer.
   interactive - Spell-check during any interactive check.
-  t           - Always spell-check."
+  t           - Always spell-check.
+
+There is a list of Lisp-specific words which checkdoc will
+install into Ispell on the fly, but only if Ispell is not already
+running.  Use `ispell-kill-ispell' to make checkdoc restart it
+with these words enabled."
   :type '(choice (const nil)
           (const defun)
           (const buffer)