Clarified tutorial context in section introducing `orm:scehma-tool:*` commnads

When I was doing the confused why the tutorial suggest running `vendor/bin/doctrine orm:schema-tool:create` when no entities exist. I somewhat suspect n00bs are getting tripped up by this too, as evidenced by this S.O. question: http://stackoverflow.com/questions/17473225/doctrine2-no-metadata-classes-to-process