Document max number of stars in clocking section

Clocking only works with all headings indented with less than `30'
stars (hardcoded `lmax' value in `org-clock-sum').

doc/org.texi

@@ -5918,12 +5918,14 @@ created for this purpose, it is described in @ref{Structure editing}.
 @cindex time clocking
 
 Org mode allows you to clock the time you spend on specific tasks in a
-project.  When you start working on an item, you can start the clock.
-When you stop working on that task, or when you mark the task done, the
-clock is stopped and the corresponding time interval is recorded.  It
-also computes the total time spent on each subtree of a project.  And it
-remembers a history or tasks recently clocked, to that you can jump quickly
-between a number of tasks absorbing your time.
+project.  When you start working on an item, you can start the clock.  When
+you stop working on that task, or when you mark the task done, the clock is
+stopped and the corresponding time interval is recorded.  It also computes
+the total time spent on each subtree@footnote{Clocking only works if all
+headings are indented with less than 30 stars.  This is a hardcoded
+limitation of `lmax' in `org-clock-sum'.} of a project.  And it remembers a
+history or tasks recently clocked, to that you can jump quickly between a
+number of tasks absorbing your time.
 
 To save the clock history across Emacs sessions, use
 @lisp