Update Collections.scala

Small documentation fixes for `unfoldLeft` method.
Merge pull request #3427 from falconepl/patch-1

Update Collections.scala - documentation fixes
Documented dependency injected routers

* Fixed a bug in routes dependency ordering
* Explained clearly the migration approach in the 2.4 migration guide
* Updated as many code samples as I could find to use dependency
  injected routers
* Documented how to use the injected router in a compile time injected
  application
* Updated all Activator templates to use dependency injected router