WeakMap::offsetGet() does not return null (#3463)

* WeakMap::offsetGet() does not return null
* adds error section

language/predefined/weakmap/offsetget.xml

@@ -34,11 +34,16 @@
  <refsect1 role="returnvalues">
   &reftitle.returnvalues;
   <para>
-   Returns the value associated to the object passed as argument, &null;
-   otherwise.
+   Returns the value associated to the object passed as argument.
   </para>
  </refsect1>
 
+ <refsect1 role="errors"><!-- {{{ -->
+  &reftitle.errors;
+  <para>
+   Throws an <classname>Error</classname> on failure.
+  </para>
+ </refsect1>
 
 </refentry>
 <!-- Keep this comment at the end of the file