Improve Content: Sending request data

[AlexanderKaran]

The API Design Guide went in a direction of "how sending requests works" before we got to really go deep on the concept of resources and collections as a whole, with that content just being kinda explained in responses but really its both.

Things to think about in some "resources and collections in general" before getting stuck right into sending requests.

• Your data model is not your API: https://tyk.io/blog/your-data-model-is-not-an-api/