Add 'clocked when-clause' to document index

chapters/synchronous.tex

@@ -811,8 +811,7 @@ \subsection{Sub-clock conversion operators}\label{sub-clock-conversion-operators
 
 \section{Clocked When-Clause}\label{clocked-when-clause}
 
-In addition to the previously discussed conditional when-clause, a
-\emph{clocked} when-clause is introduced:
+In addition to the previously discussed conditional when-clause, a \emph{clocked} when-clause\index{clocked!when-clause}\index{when-clause!clocked} is introduced:
 \begin{lstlisting}[language=modelica,escapechar=!]
 when $\mathit{clockExpression}$ then
   !\emph{\textless{}clocked equations\textgreater{}}!