Update purity exception for Streams.print

Reformulating to match the current MSL where 'print' is declared impure.

Author: henrikt-ma

chapters/functions.tex

@@ -290,11 +290,13 @@ \section{Pure Modelica Functions}\label{pure-modelica-functions}
   Binding equations for external objects.
 \end{itemize}
 
+An exception is made for \lstinline!Modelica.Utilities.Streams.print!, for which none of the impure function restrictions apply.
+
 It is an error if an impure function call is part of a systems of equations (including linear systems), even if called in agreement with the restrictions above.
 The reason is that solving systems of equations generally requires expressions to be evaluated an unknown number of times.
 This includes the special handling of \lstinline!when initial()! during initialization.
 
-There are two ways in which an impure function could be called in a system of equations, namely in the deprecated case of external functions assumed to be impure, and when using \lstinline!pure($\ldots$)! to call an impure function from within a pure function.
+There are three ways in which an impure function could be called in a system of equations, namely in the deprecated case of external functions assumed to be impure, when using \lstinline!pure($\ldots$)! to call an impure function from within a pure function, and the exception for \lstinline!Modelica.Utilities.Streams.print!.
 The side-effect semantics of the function call are then undefined.
 Specifically, the number of calls with external side-effects is unspecified.
 However, for impure functions where the outputs only depend on the inputs the system of equations should be solved correctly.