Document functionality of /prices endpoint

[mattiaspalmqvist]

I can't find any documentation of how the results in /prices endpoint is supposed to work. What I can remember is that we discussed that is shall provide as many actual prices as possible, and after the last actual price the preview prices fills the answer up untill toExcluding in the API call or as many preview prices that are available.

[mattiaspalmqvist]

I created a commit that documents this behavior. Please have a look.

We could also leave the implementation of actual/preview more open and make it up to each DSO what data should be presented. It will be a bit more confusing for the user, but leaves the implementation specifics more open for the DSO.