Clarified tutorial context around introduction of `orm:schema-tool:*` commands

See: http://stackoverflow.com/questions/17473225/doctrine2-no-metadata-classes-to-process for evidence of this confusion
further tweaking of last commit
further tweaking of last commit pt.2
Merge pull request #818 from mfdj/master

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