Documentation fix, (find returns promises)

I tried to find all occurences when find was used as if it returned a post object and not a promise, but there might be some ones left.

Maybe the lines that look like this: 
```js
store.find('post', 1).then(function (post) {
});
```

Would benefit from some more explanation of the fact that it returns a promise object, but I'm not sure how one would do that.
@Hatlen Do you mind also submitting a pr making this change to the JSDoc comments in [the ember data repo](https://github.com/emberjs/data/blob/d0a86e40f89403e4d3f641e57789e368b6622781/packages/ember-data/lib/system/store.js#L71-L78)? That way when `data/data_api.yml` is regenerated from the source this change can be preserved.