Move explanation of what a keyword is to chapter 'Lexical Structure'

Makes the introducing easier to digest, and places explanation where one would expect it.

chapters/introduction.tex

@@ -104,6 +104,5 @@ \section{Notation and Grammar}\label{notation-and-grammar}
 The meta symbols (of the extended BNF-grammar) are defined in~\cref{lexical-conventions}.
 
 In Modelica code listings, keywords are highlighted with color and may come in both normal weight and boldface, for example, \lstinline!connect! and \lstinline!equation!.
-Keywords are reserved words and shall not be used as identifiers, with the exception of \lstinline!initial! which is a keyword in section headings, and \lstinline!der! which is a keyword for declaration functions, but it is also possible to call the functions \lstinline!initial! and \lstinline!der!.
 
 See \cref{modelica-concrete-syntax} for a full lexical specification and grammar.

chapters/lexicalstructure.tex

@@ -108,7 +108,7 @@ \subsection{Names}\label{names}
 
 \subsection{Modelica Keywords}\label{modelica-keywords}
 
-The following Modelica \firstuse{keywords}\index{keyword} are reserved words and shall not be used as identifiers, except as listed in \cref{lexical-conventions}:
+The following Modelica \firstuse{keywords}\index{keyword} are reserved words and shall not be used as identifiers, with the exception of \lstinline!initial! which is a keyword in section headings, and \lstinline!der! which is a keyword for declaration functions, but it is also possible to call the functions \lstinline!initial! and \lstinline!der!:
 \begin{center}
 \begin{tabular}{l l l l l}
 \lstinline!algorithm! & \lstinline!discrete! & \lstinline!false! & \lstinline!loop! & \lstinline!pure!\\ \hline