Document the fourth parameter of spawn() in the language reference

Also clarify that the defaults are based on the corresponding sizes for
the main thread (rather than all being based on heap size).

doc/book/langref.tex

@@ -3332,14 +3332,14 @@ \subsection*{Built-in functions}
 \bigskip\hrule\vspace{0.1cm}
 \index{spawn}
 \noindent
-{\bf spawn(CE, i, i) : thread } \hfill {\bf launch asynchronous thread}
+{\bf spawn(CE, i, i, i) : thread } \hfill {\bf launch asynchronous thread}
 
 \noindent
 \index{spawn()}\texttt{spawn(ce)} launches co-expression \texttt{ce} as an
 asynchronous thread that will execute concurrently with the current
-co-expression. The two optional integers specify the memory in bytes
-allocated for the thread's block and string regions. The defaults are 10\%
-of the main thread heap size.
+co-expression. The three optional integers specify the memory in bytes
+allocated for the thread's block and string regions and stack size.
+The defaults are 10\% of the corresponding sizes for the main thread.
 
 \bigskip\hrule\vspace{0.1cm}
 \index{sql}