DOC: slice_indexer correction + examples
The stated return value of ``Index.slice_indexer`` in the doc string is wrong, the method can only return slices. Also added some examples.

In addition, some very minor corrections on the user guide text on ``IntervalIndex`` (moved the versionadded to the chapter top +  a spelling correction).  EDIT: These are not related to the ``.slice_indexer`` changes, but are attached to this PR as the changes are very small.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17829?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@390f36e`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17829/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17829?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #17829   +/-   ##
=========================================
  Coverage          ?   91.22%           
=========================================
  Files             ?      163           
  Lines             ?    49994           
  Branches          ?        0           
=========================================
  Hits              ?    45606           
  Misses            ?     4388           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.02% <ø> (?)` | |
| #single | `40.24% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17829?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/17829?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.47% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17829?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/17829?src=pr&el=footer). Last update [390f36e...bb8050a](https://codecov.io/gh/pandas-dev/pandas/pull/17829?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/17829?src=pr&el=h1) Report
> Merging [#17829](https://codecov.io/gh/pandas-dev/pandas/pull/17829?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a355ed2a8c32a29c1dc93466f9b65870cac272fa?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17829/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17829?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17829      +/-   ##
==========================================
- Coverage   91.26%   91.22%   -0.05%     
==========================================
  Files         163      163              
  Lines       50099    50099              
==========================================
- Hits        45724    45703      -21     
- Misses       4375     4396      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.24% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17829?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/17829?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.42% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17829?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17829?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17829?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17829?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/17829?src=pr&el=footer). Last update [a355ed2...e78f3cd](https://codecov.io/gh/pandas-dev/pandas/pull/17829?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Ok, I've changed according to the suggestions.

I think the warning is ok, because I'm a bit surprised that it's even possible to slice a non-ordered unique index; maybe you'd want that some times, but I bet 9 out of 10 times, someone who slices a index expects it to be ordered. @topper-123 Thanks!