Forbid noClock(Clock(1))

Closes modelica#2355

chapters/synchronous.tex

@@ -696,7 +696,7 @@ \subsection{Sub-clock conversion operators}\doublelabel{sub-clock-conversion-ope
 Clock. {[}\emph{The operators can operate on all types of clocks}{]}. If
 u is a clocked expression, the operator returns a clocked variable that
 has the same type as the expression. If u is an expression of type
-Clock, the operator returns a Clock.
+Clock, the operator returns a Clock - except for noClock where it is an error.
 
 The optional input arguments \lstinline!factor! (default=0, min=0), and \lstinline!resolution!
 (default=1, min=1) are parameter expressions of type Integer.