Document INVISIBLE-OK argument of `org-forward-same-level'

* lisp/org.el (org-forward-same-level): Fix docstring.

lisp/org.el

@@ -19303,7 +19303,9 @@ If there is no such heading, return nil."
 
 (defun org-forward-same-level (arg &optional invisible-ok)
   "Move forward to the arg'th subheading at same level as this one.
-Stop at the first and last subheadings of a superior heading."
+Stop at the first and last subheadings of a superior heading.
+Normally this only looks at visible headings, but when INVISIBLE-OK is non-nil
+it wil also look at invisible ones."
   (interactive "p")
   (org-back-to-heading invisible-ok)
   (org-on-heading-p)