Builder description is inaccurate

[krancour]

The following describes builder inaccurately.

https://github.com/deis/docs-v2/blame/master/src/understanding-deis/components.md#L23-L29

Among the inaccuracies-- in the case of buildpack apps, builder does not produce Docker images, but rather slugs, and builder does not incorporate last-mile config into the applications.