Documentation missing for /api/contacts/:id/avatar

I found monicahq/monica#3302 and then the avatar test files, and so I managed to figure out that the API looks like:

```
PUT /api/contacts/:id/avatar
{
source: "photo",
photo_id: 1
}
```

Maybe there are also other changes from monicahq/monica#3302 which need added to the docs? I'd be open to looking into submitting a PR. Just let me know if that would be appreciated.