Clarify rules about allowed combination of String named arguments

chapters/operatorsandexpressions.tex

@@ -501,21 +501,24 @@ \subsection{Numeric Functions and Conversion Functions}\label{numeric-functions-
 \end{lstlisting}\end{synopsis}
 \begin{semantics}
 Convert a scalar non-\lstinline!String! expression to a \lstinline!String! representation.
-The first argument may be a \lstinline!Boolean! $b$, an \lstinline!Integer! $i$, a \lstinline!Real! $r$ or an enumeration value $e$ (\cref{type-conversion-of-enumeration-values-to-string-or-integer}).
-The other arguments must use named arguments.
-For \lstinline!Real! expressions the output shall be according to the Modelica grammar.
-
-The optional \lstinline!<options>! are:
+The first argument may be a \lstinline!Boolean! $b$, an \lstinline!Integer! $i$, a \lstinline!Real! $r$, or an enumeration value $e$ (\cref{type-conversion-of-enumeration-values-to-string-or-integer}).
+The other arguments can only be passed as named arguments, and the recognized ones are:
 \begin{itemize}
+\item
+  \lstinline!String format!: ANSI-C style format string, see below.
+  Only allowed for formatting of a \lstinline!Real! value, and must not be combined with other named arguments.
 \item
   \lstinline!Integer minimumLength = 0!: Minimum length of the resulting string.
   If necessary, the blank character is used to fill up unused space.
 \item
   \lstinline!Boolean leftJustified = true!: If true, the converted result is left justified in the string; if false it is right justified in the string.
 \item
   \lstinline!Integer significantDigits = 6!: Number of significant digits in the result string.
+  Required when formatting a \lstinline!Real! value and \lstinline!format! is not provided.
 \end{itemize}
 
+For \lstinline!Real! expressions the output shall be according to the Modelica grammar.
+
 \begin{nonnormative}
 Examples of \lstinline!Real! values formatted with 6 significant digits: \emph{12.3456}, \emph{0.0123456}, \emph{12345600}, \emph{1.23456E-10}.
 \end{nonnormative}