DOC: Add examples to MultiIndex.slice_locs + note that index.slice requires
Clarify ``.slice_locs`` and set up examples for how to use the method.

Also, for ``pd.Index`` clarify that using ``.slice_locs`` requires that the index is monotonic.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17799?src=pr&el=h1) Report
> Merging [#17799](https://codecov.io/gh/pandas-dev/pandas/pull/17799?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5653fe41b41fa1b60f3a7edf88a47f29ec7868ee?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/17799/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17799?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17799      +/-   ##
==========================================
- Coverage   91.24%    91.2%   -0.05%     
==========================================
  Files         163      163              
  Lines       49971    49971              
==========================================
- Hits        45597    45576      -21     
- Misses       4374     4395      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.24% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17799?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/17799?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `96.39% <ø> (ø)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/17799?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.46% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17799?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17799?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/17799?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.74% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17799?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17799?src=pr&el=footer). Last update [5653fe4...d41388b](https://codecov.io/gh/pandas-dev/pandas/pull/17799?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Commit added doing the requested changes. @topper-123 Thanks!