Document max number of stars in headings in docstring of org-inlinetask-minlevel

Clocking only works with headings indented with less than `30' stars
(hardcoded `lmax' value in `org-clock-sum').  Since especially inline
tasks may dupe someone into using more stars, document the limit in
the docsring of `org-inlinetask-min-level'.

lisp/org-inlinetask.el

@@ -90,6 +90,9 @@
 
 (defcustom org-inlinetask-min-level 15
   "Minimum level a headline must have before it is treated as an inline task.
+Don't set it to something higher than `29' or clocking will break since this 
+is the hardcoded maximum number of stars `org-clock-sum' will work with.
+
 It is strongly recommended that you set `org-cycle-max-level' not at all,
 or to a number smaller than this one.  In fact, when `org-cycle-max-level' is
 not set, it will be assumed to be one less than the value of smaller than