Issue #3199742 by Matroskeen, ravi.shankar, quietone: Add documentation for block source plugins

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