Issue #2435231 by mikeryan: Documented and tweaked

migrate_example/README.txt

...example/config/install/migrate.migration.beer_comment.yml

...te_example/config/install/migrate.migration.beer_node.yml

...te_example/config/install/migrate.migration.beer_term.yml

...te_example/config/install/migrate.migration.beer_user.yml

...mple/config/install/migrate_plus.migration_group.beer.yml

migrate_example/migrate_example_setup/beer.install.inc

migrate_example/src/Plugin/migrate/source/BeerComment.php

migrate_example/src/Plugin/migrate/source/BeerNode.php

migrate_example/src/Plugin/migrate/source/BeerTerm.php

migrate_example/src/Plugin/migrate/source/BeerUser.php

migrate_plus.module