bug#10392: mention ad-handle-definition startup warnings

[jidanni]

X-Debbugs-Cc: yamaoka@jpl.org

(info "(elisp) Advising Functions")
doesn't mention that e.g., putting

(defadvice gnus-summary-select-article-buffer (after jump-to-html-part
                                                     activate)
  "Jump to an html part that emacs-w3m rendered, if any."
  (unless (get-text-property (point) 'mm-inline-text-html-with-w3m)
    (let ((pos (text-property-any (point-min) (point-max)
                                  'mm-inline-text-html-with-w3m t)))
      (gnus-goto-char pos))))

in .gnus.el will cause startup warnings

   ad-handle-definition: `gnus-summary-select-article-buffer' got redefined

for the rest of one's life, with no mention of workarounds to turn them
off either.