Issue #3225227 by Matroskeen, quietone, Kristen Pol: Fix source plugin documentation

core/modules/block/src/Plugin/migrate/source/Block.php

...s/block/src/Plugin/migrate/source/d6/BlockTranslation.php

...s/block/src/Plugin/migrate/source/d7/BlockTranslation.php

...odules/block_content/src/Plugin/migrate/source/d6/Box.php

...k_content/src/Plugin/migrate/source/d6/BoxTranslation.php

...lock_content/src/Plugin/migrate/source/d7/BlockCustom.php

...t/src/Plugin/migrate/source/d7/BlockCustomTranslation.php

.../src/Plugin/migrate/source/d6/ProfileFieldTranslation.php

core/modules/field/src/Plugin/migrate/source/d6/Field.php

...ules/field/src/Plugin/migrate/source/d6/FieldInstance.php

...ugin/migrate/source/d6/FieldInstanceOptionTranslation.php

.../Plugin/migrate/source/d6/FieldInstancePerFormDisplay.php

...src/Plugin/migrate/source/d6/FieldInstancePerViewMode.php

...in/migrate/source/d6/FieldLabelDescriptionTranslation.php

...d/src/Plugin/migrate/source/d6/FieldOptionTranslation.php

core/modules/field/src/Plugin/migrate/source/d7/Field.php

...ules/field/src/Plugin/migrate/source/d7/FieldInstance.php

.../Plugin/migrate/source/d7/FieldInstancePerFormDisplay.php

...src/Plugin/migrate/source/d7/FieldInstancePerViewMode.php

...in/migrate/source/d7/FieldLabelDescriptionTranslation.php