In order to render a suitable frontend URI, this function strips off the path to the site node and only keeps
the actual node path relative to that site node. In practice this function would set $this->value as follows:
absolute node path: /sites/neostypo3org/homepage/about
$this->value: homepage/about
absolute node path: /sites/neostypo3org/homepage/about@user-admin
$this->value: homepage/about@user-admin