Update API Documentation

[oscgonfer]

This is an issue to list developer documentation items that need to be updated:

• [x] Add a device requires token, not MAC_ADDRESS https://developer.smartcitizen.me/?shell#add-a-device
• [x] World map doesn't return last readings anymore: https://developer.smartcitizen.me/?shell#world-map-of-devices

[oscgonfer]

Overall I believe that a good review of the docs is necessary. We will do so after devices refactor!

[oscgonfer]

Tracking things to be updated here:

• [x] websockets
• [x] devices model

[oscgonfer]

@timcowlishaw some additional checks if you don't mind going through them:

• [ ] Device Add / update (and double check I did world_map correctly)
• [ ] User Add / update
• [ ] Sensor Add / update
• [ ] Measurement Add / update
• [ ] Experiment in general
• [ ] Auth rewording for preciseness