update error 'message' description to match new numeric default

website/guide/errorobjects.html

@@ -42,6 +42,13 @@ objects is minimized to keep error objects as small as possible.</p>
     Error object.</li>
 </ul>
 
+<p>When error objects are created using the Duktape API from C code and the
+caller does not give a format string for a <tt>message</tt>, the <tt>message</tt>
+property is set to a numeric error code given in the API call.  The type of
+<tt>message</tt> will be number in this case; normally error messages are
+strings.   In minimized Duktape builds all errors generated internally by
+Duktape use numeric error codes only.</p>
+
 <p>An object is considered an "error object" if its internal prototype
 chain contains the (original) <tt>Error.prototype</tt> object.  Only
 objects matching this criteria get augmented with e.g. traceback data.</p>
@@ -82,6 +89,11 @@ URIError: invalid input
         global ecmascript-testcases/test-dev-traceback-example.js:3
 </pre>
 
+<p>In builds where tracebacks are disabled, the <tt>stack</tt> accessor
+will return the same value as calling <tt>toString()</tt> on the error
+would.  This means you can always print <tt>e.stack</tt> and get a useful
+output.</p>
+
 <p>Attempt to write to <tt>stack</tt> is silently ignored.  You can still
 override the accessor by defining an own property of the same name explicitly
 with <tt>Object.defineProperty()</tt>.  This behavior differs from V8 where