Issue #1418980 by batigolix, kotnik, hansyg, adnasa: API docs cleanup for RDF module

...odules/rdf/lib/Drupal/rdf/Tests/CommentAttributesTest.php

core/modules/rdf/lib/Drupal/rdf/Tests/CrudTest.php

core/modules/rdf/lib/Drupal/rdf/Tests/GetNamespacesTest.php

...modules/rdf/lib/Drupal/rdf/Tests/GetRdfNamespacesTest.php

...odules/rdf/lib/Drupal/rdf/Tests/MappingDefinitionTest.php

core/modules/rdf/lib/Drupal/rdf/Tests/MappingHookTest.php

core/modules/rdf/lib/Drupal/rdf/Tests/RdfaMarkupTest.php

core/modules/rdf/rdf.module

core/modules/rdf/tests/rdf_test.module