users_guide: Document zero stack size limit

docs/users_guide/runtime_control.xml

@@ -679,7 +679,8 @@ $ ./prog -f +RTS -H32m -S -RTS -h foo bar
 	  <para>&lsqb;Default: 8M&rsqb; Set the maximum stack size for
           an individual thread to <replaceable>size</replaceable>
           bytes.  If the thread attempts to exceed this limit, it will
-            be send the <literal>StackOverflow</literal> exception.
+          be sent the <literal>StackOverflow</literal> exception. The
+          limit can be disabled entiredly by specifying a size of zero.
           </para>
           <para>
             This option is there mainly to stop the program eating up