Clarify that clock is only determined from equations. (modelica#3283)

* Clarify that clock is only determined from equations.
Closes modelica#3265
Co-authored-by: Henrik Tidefelt <henrikt@wolfram.com>

chapters/synchronous.tex

@@ -882,6 +882,11 @@ \section{Clock Partitioning}\label{clock-partitioning}
 Note that incidences of the first argument of clock conversion operators
 of \cref{partitioning-operators} are handled specially.
 
+\begin{nonnormative}
+As clock partitions are solely determined by the equations, two different clock partitions can have clocks defined by the same expressions.
+It is a quality of implementation issue that such partitions are executed synchronously, e.g., by putting them in the same task in a real-time simulation context.
+\end{nonnormative}
+
 \subsection{Flattening of Model}\label{flattening-of-model}
 
 The clock partitioning is conceptually performed after model flattening,