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)



# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=h1) Report
> Merging [#24780](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6d3565a5181b3d0fb8342e69dd1d21a98ac3d380?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24780/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24780      +/-   ##
==========================================
+ Coverage   92.38%   92.38%   +<.01%     
==========================================
  Files         166      166              
  Lines       52363    52366       +3     
==========================================
+ Hits        48377    48380       +3     
  Misses       3986     3986
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <100%> (ø)` | :arrow_up: |
| #single | `42.91% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/24780/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2ludGVydmFsLnB5) | `93.12% <100%> (+0.03%)` | :arrow_up: |
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/24780/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `95.27% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=footer). Last update [6d3565a...6af067b](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=h1) Report
> Merging [#24780](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6d3565a5181b3d0fb8342e69dd1d21a98ac3d380?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24780/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24780      +/-   ##
==========================================
+ Coverage   92.38%   92.38%   +<.01%     
==========================================
  Files         166      166              
  Lines       52363    52366       +3     
==========================================
+ Hits        48377    48380       +3     
  Misses       3986     3986
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <100%> (ø)` | :arrow_up: |
| #single | `42.91% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/24780/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2ludGVydmFsLnB5) | `93.12% <100%> (+0.03%)` | :arrow_up: |
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/24780/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `95.27% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=footer). Last update [6d3565a...6af067b](https://codecov.io/gh/pandas-dev/pandas/pull/24780?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks