DOC: IntervalArray and IntervalIndex minor doc fixes
Some minor fixes for things that aren't rendering correctly for `IntervalArray` and `IntervalIndex`.  I've built the docs locally and the changes look good.

For [`IntervalArray` ](https://pandas-docs.github.io/pandas-docs-travis/api/generated/pandas.IntervalArray.html):
- `IntervalArray.values` doesn't exist, so removing it.

![image](https://user-images.githubusercontent.com/5332445/51159115-bf6dd600-1844-11e9-9d0f-ec5a1e132a5f.png)

For [`IntervalIndex`](https://pandas-docs.github.io/pandas-docs-travis/api/generated/pandas.IntervalIndex.html):
- `IntervalIndex.is_non_overlapping_monotonic` lost it's docstring, so adding it back via shared docs
- `IntervalIndex.to_tuples` isn't being generated, so now generating it. Also fixed versionadded directive.

![image](https://user-images.githubusercontent.com/5332445/51159221-520e7500-1845-11e9-8633-fdfc5d646d52.png)


