Clarify documentation for dynamic segment identifiers

* Right now the guide docs don't make it very clear that you can pass either a model(object) or an identifier(string/number) to a segment when transitioning to a new route.  It can be really handy at times.  The link-to helper documentation mentions it but only briefly. I changed one of the examples to make it more obvious.   

* The link-to documentation is very helpful to understand how `transitionTo` uses its parameters and how to transition in more complicated situations.  I added some wordage that points people to the `link-to` docs.