set_error_handler() no longer returns null on failure

As of PHP 8.0.0, it throws a TypeError; prior to that version, that was
undefined behavior (as documented for internal functions).

reference/errorfunc/functions/set-error-handler.xml

@@ -172,8 +172,8 @@
   &reftitle.returnvalues;
   <para>
    Returns the previously defined error handler (if any). If
-   the built-in error handler is used &null; is returned. &null; is also returned
-   in case of an error such as an invalid callback. If the previous error handler
+   the built-in error handler is used &null; is returned.
+   If the previous error handler
    was a class method, this function will return an indexed array with the class
    and the method name.
   </para>