DOC: update multi-index term with MultiIndex
Hello,
before I start working on part 2 of the sphinx spelling extension I wanted to submit this PR in order to fix the double use of multi-index and MultiIndex. I have changed all `multi-index` mentions through the documentation to `MultiIndex`, this meant that some titles needed changing and new linking needed to be done. I have tested the documentation locally and everything seems to be working okay.

I have also reverted the changes to some adverbs that got changed previously (which shouldn't have according to @h-vetinari)

I hope that creating this PR before working on part 2 is okay. If you would rather me add this and the next one together let me know and I'll update the title and whatnot 😄 👍 


- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21382?src=pr&el=h1) Report
> Merging [#21382](https://codecov.io/gh/pandas-dev/pandas/pull/21382?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/abfac97b2d22447d41bfccaa53e0a264ca34d6d4?src=pr&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21382/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21382?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21382      +/-   ##
==========================================
+ Coverage   91.85%   91.88%   +0.03%     
==========================================
  Files         153      153              
  Lines       49570    49570              
==========================================
+ Hits        45533    45549      +16     
+ Misses       4037     4021      -16
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.28% <ø> (+0.03%)` | :arrow_up: |
| #single | `41.86% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21382?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/21382/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `83.53% <0%> (+1.14%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21382?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/21382?src=pr&el=footer). Last update [abfac97...767485d](https://codecov.io/gh/pandas-dev/pandas/pull/21382?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @FabioRosado : Thanks!