documenting optional string argument of display and error

doc/source_names.tex

@@ -54,8 +54,11 @@ \section*{Names}
 the ``OK'' button is pressed, \texttt{prompt} returns the entered text as a string.
 If the ``Cancel'' button is pressed, \texttt{prompt} returns a non-string value.
 \item \href{https://sicp.comp.nus.edu.sg/chapters/17\#footnote-6}{\texttt{display(x)}}: Displays the value \texttt{x} in the console\footnote{The notation used for the display of values is consistent with \href{http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf}{\color{DarkBlue}JSON}, but also displays \texttt{undefined} and function objects.}; returns the argument \texttt{a}.  
+\item \texttt{display(x, s)}: Displays the string \texttt{s}, followed by a space character, followed by the value \texttt{x} in the console\footnotemark[\value{footnote}]; returns the argument \texttt{x}.
 \item \href{https://sicp.comp.nus.edu.sg/chapters/21\#footnote-3}{\texttt{error(x)}}: Displays the value \texttt{x} in the console\footnotemark[\value{footnote}] with error flag. The evaluation
   of any call of \texttt{error} aborts the running program immediately.
+\item \texttt{error(x, s)}: Displays the string \texttt{s}, followed by a space character, followed by the value \texttt{x} in the console\footnotemark[\value{footnote}] with error flag. The evaluation
+  of any call of \texttt{error} aborts the running program immediately.
 \item \href{https://sicp.comp.nus.edu.sg/chapters/62}{\lstinline{stringify(x)}}: returns a string that represents\footnotemark[\value{footnote}] the value \texttt{x}. 
 \end{itemize}
 All Source primitive functions, except \verb#stringify#, can be assumed to run