Add special text for sample in clocked systems. (modelica#3099)

* Add special text for sample in clocked systems.
Revert "RemoveSampleNow"

This reverts commit 32770c0.

Co-authored-by: Henrik Tidefelt <henrikt@wolfram.com>

chapters/synchronous.tex

@@ -1092,6 +1092,14 @@ \subsection{Clocked Discrete-Time and Discretized Continuous-Time}\label{clocked
 Such a partition has to be solved with a \emph{solver method} of \cref{solver-methods}.
 When \lstinline!previous(x)! is used on a continuous-time state variable \lstinline!x!, then \lstinline!previous(x)! uses the start value of \lstinline!x! as value for the first clock tick.
 
+The use of the operator \lstinline!sample! from \cref{event-related-operators-with-function-syntax} in a clocked partition is problematic.
+A diagnostic is recommended, especially if the operator is intended to generate events faster than the clock ticks, and otherwise the sampling should ideally be adjusted to the clock ticks.
+
+\begin{nonnormative}
+The reason for not disallowing \lstinline!sample! in a clocked partition is to make it possible to include \emph{any} continuous-time Modelica model in a sampled data control system.
+Note that even if the sampling is slower than the clock ticks (or even the same rate) it still introduces the problem of possibly uneven sampling.
+\end{nonnormative}
+
 In a clocked discrete-time partition all event generating mechanisms do no longer apply.
 Especially neither relations, nor any of the built-in operators of \cref{event-triggering-mathematical-functions} (event triggering mathematical functions) will trigger an event.