Sample API Usage page is a bit confusing about client

[mattmoor]

We should consider moving this to the clients repo? cc @sixolet

[maximilien]

I am also happy to submit a PR to the serving API docs to get it up to date with kn --- at least everywhere we can so far. Let me know preferred approach? Move all content to this repo as @mattmoor suggests or keep separate and fix. I am cool with either.

[rhuss]

As https://github.com/knative/serving/blob/master/docs/spec/normative_examples.md has been replaced by a pointer to the knative client githup repo, I think we can close this issue. The examples that have been used included in this document will be moved more less over to the forthcoming user manual (#650)

btw, I think https://github.com/knative/serving/blob/master/docs/spec/normative_examples.md could be completely removed as it looks like that it is not referenced from anywhere