More clearly define that fallfack value is applicable everywhere.

chapters/classes.tex

@@ -1398,8 +1398,8 @@ \section{Predefined Types and Classes}\label{predefined-types-and-classes}
 These are called the \firstuse[primitive type]{primitive types}.
 
 \begin{definition}[Fallback value]\label{def:fallback-value}\index{fallback value}
-When the start-value is needed and the \lstinline!start!-attribute is not provided, the fallback value is used instead.
-The fallback value can be be used as guess-value in system of equations and when initializing variables in algorithm sections.
+In situations where the \lstinline!start!-attribute would apply if provided, but the attribute is not provided, the \emph{fallback} shall be used instead.
+Tools are recommended to give diagnostics when the fallback value is used.
 The fallback values for variables of the different predefined types are defined below.
 \end{definition}