Document sparse array behavior with {{#each

Hey!

I just noticed that Ember 2.10 introduces a changed behaviour when looping over indexed arrays with unset indexes.

Let's assume this array:

```js
let a = [];
a[5] = 'foo';
a[6] = 'bar';
```

and this hbs:

```hbs
{{#each testArray as |v k|}}{{k}},{{/each}}
```

In 2.9 this produces `5,6` and in 2.10 it produces `0,1,2,3,4,5,6`.

I also made a [twiddle](https://ember-twiddle.com/5e7d30c69358b5094ae644ddeaf9f947) for that

Start with the linked Ember Twiddle and reproduce the sparse-array example using the shown JavaScript array and Handlebars each loop. Document the differing Ember 2.9 and 2.10 output, including how unset indexes are handled; the work is done when this behavior is clearly recorded for users.

Written by the indexing model from the issue text.