Add Documentation for getTemplateSize() method

[MarcelBolten]

https://github.com/mpdf/mpdf/blob/3b21745a7b0178c14bf512d79b790a4351c16bce/src/FpdiTrait.php#L283-L286

public function getTemplateSize($tpl, $width = null, $height = null)
{
    return $this->getImportedPageSize($tpl, $width, $height);
}