Clarify caption: the value of a clock variable isn't defined

chapters/synchronous.tex

@@ -230,7 +230,7 @@ \subsection{Clocks and Clocked Variables}\doublelabel{clocks-and-clocked-variabl
   \begin{center}
     \includegraphics[width=3in]{clock}
   \end{center}
-  \caption{A clock variable.}\doublelabel{fig:clock-variable}
+  \caption{A clock variable.  The value of a clock variable is not defined --- the plot marks only indicate \emph{when} the clock is active.}\doublelabel{fig:clock-variable}
 \end{figure}
 
 \begin{definition}[Clocked variable]