add REST documentation for principals module

This looks like the output from the `me` feed, so presumably you were either using the current user's userid or were performing the request as a manager. As a logged in user, when you do `/api/user/:id` using a different user's userid, you get something like this:

```
{
    "tenant": {
        "alias": "oae",
        "displayName": "Open Academic Environment"
    },
    "id": "u:oae:-ydVfUXUb",
    "displayName": "Aretha Veillette",
    "visibility": "loggedin",
    "picture": {
        "small": "/api/download/signed?uri=local%3Au%2Foae%2F-y%2FdV%2FfU%2FXU%2F-ydVfUXUb%2Fprofilepictures%2F1407384413865%2Fsmall.jpg&expires=1408579200000&signature=9b3aae3e94d96946fea271c17f70b3dbbc1ce65b",
        "medium": "/api/download/signed?uri=local%3Au%2Foae%2F-y%2FdV%2FfU%2FXU%2F-ydVfUXUb%2Fprofilepictures%2F1407384413865%2Fmedium.jpg&expires=1408579200000&signature=be7cb52c455b32434a79d3a62385734b95c507d0",
        "large": "/api/download/signed?uri=local%3Au%2Foae%2F-y%2FdV%2FfU%2FXU%2F-ydVfUXUb%2Fprofilepictures%2F1407384413698%2Flarge.jpg&expires=1408579200000&signature=0e33b679ed8dc73e2ad1d646cffbd978d33631c0"
    },
    "profilePath": "/user/oae/-ydVfUXUb",
    "resourceType": "user",
    "lastModified": 1407384987987,
    "following": {
        "canFollow": true,
        "isFollowing": false
    }
}
```