Issue #3271313: Improve documentation for classes

src/Component/Helper/FillPdfMappingHelper.php

src/Component/Helper/FillPdfMappingHelperInterface.php

src/Component/Utility/FillPdf.php

src/Controller/HandlePdfController.php

src/EntityHelper.php

src/EntityHelperInterface.php

src/ExportableContentEntityInterface.php

src/FillPdfAccessHelper.php

src/FillPdfAccessHelperInterface.php

src/FillPdfAdminFormHelperInterface.php

src/FillPdfFormFieldInterface.php

src/FillPdfFormFieldViewsData.php

src/FillPdfFormInterface.php

src/FillPdfFormViewsData.php

src/Form/FillPdfAdminFormBase.php

src/Form/FillPdfFormConfirmFormBase.php

src/InputHelper.php

src/InputHelperInterface.php

src/OutputHandler.php

src/Serializer.php