Allow finding 'short class definition' in the document index

In order to make the index entry point to a good location, I needed to rephrase the paragraph slightly.  I'd say breaking the sentence in two, like this, was good for readability anyway.

chapters/classes.tex

@@ -747,11 +747,11 @@ \section{Class Declarations}\label{class-declarations}
 
 \subsection{Short Class Definitions}\label{short-class-definitions}
 
-A class definition of the form
+A \firstuse{short class definition} is a class definition in the form
 \begin{lstlisting}[language=modelica]
 class IDENT1 = type-specifier class-modification;
 \end{lstlisting}
-is identical, except that \lstinline!type-specifier! (the base-class) may be replaceable and for the lexical scope of modifiers, where the short class definition does not introduce an additional lexical scope for modifiers, to the longer form
+Except that \lstinline!type-specifier! (the base-class) may be replaceable, and that the short class definition does not introduce an additional lexical scope for modifiers, it is identical to the longer form
 \begin{lstlisting}[language=modelica]
 class IDENT1
   extends type-specifier class-modification;