@return self correction

Different @return blocks, lead to different code completion currently.

At `$form->add()` i thought i get the current created fieldset returned, but i got the form...so circular call ...

    /**
     * Add an element or fieldset
     *
     * If $elementOrFieldset is an array or Traversable, passes the argument on
     * to the composed factory to create the object before attaching it.
     *
     * $flags could contain metadata such as the alias under which to register
     * the element or fieldset, order in which to prioritize it, etc.
     *
     * @param  array|Traversable|ElementInterface $elementOrFieldset
     * @param  array                              $flags
     * @return \Zend\Form\Fieldset|\Zend\Form\FieldsetInterface|\Zend\Form\FormInterface
     */
    public function add($elementOrFieldset, array $flags = array())
Merge branch 'cs/#6686-zend-form-return-self-fixes'

Close #6686