Update the Spotify Developer API docs

[br3tt_lee]

I've been playing around with Spotify API and found a few of the Playback APIs that are slightly incorrectly documented. The following are what I have found so far:

• Skip to Next &
• Skip to Previous &
• Pause playback &
• Play / Resume playback

These all state in the docs that they return Status Code 204 when executed. However, they actually return Staus Code 200 when executed. I'm not sure how many others are not correctly documented but it is safe to assume all endpoints will return Status Code 200