Add extra clarification for nested resources

Taken from mavilein's comments on Stack Overflow: http://stackoverflow.com/questions/14973556/what-is-the-difference-between-a-route-and-resource-in-new-router-api
Update defining-your-routes with better wording

Lines 375-377 received some word-smithing.
Merge pull request #1708 from jdhines/patch-1

Add extra clarification for nested resources