DOC: whatsnew & linked edits
small edits for consistency in the whatsnew; some incorrect links & section headings.

@datapythonista @TomAugspurger  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24649?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@dc91f4c`). [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/24649/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24649?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #24649   +/-   ##
=========================================
  Coverage          ?   43.02%           
=========================================
  Files             ?      166           
  Lines             ?    52380           
  Branches          ?        0           
=========================================
  Hits              ?    22534           
  Misses            ?    29846           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #single | `43.02% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24649?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/24649?src=pr&el=footer). Last update [dc91f4c...66a0128](https://codecov.io/gh/pandas-dev/pandas/pull/24649?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/24649?src=pr&el=h1) Report
> Merging [#24649](https://codecov.io/gh/pandas-dev/pandas/pull/24649?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/48c3ce5b5b89fa53b3d4ac9c23d4c22da5e86493?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24649/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24649?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24649   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files         166      166           
  Lines       52384    52384           
=======================================
  Hits        48390    48390           
  Misses       3994     3994
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43.02% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24649?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/24649?src=pr&el=footer). Last update [48c3ce5...c1e3ab4](https://codecov.io/gh/pandas-dev/pandas/pull/24649?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
