DOC: fixup old whatsnew for dtype coercing change (#21456)
xref https://github.com/pandas-dev/pandas/pull/21456

cc @gfyoung 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21634?src=pr&el=h1) Report
> Merging [#21634](https://codecov.io/gh/pandas-dev/pandas/pull/21634?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6db8076047f547a85038ea601dfc60bc51b4afc6?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/21634/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/21634?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21634   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         153      153           
  Lines       49547    49547           
=======================================
  Hits        45536    45536           
  Misses       4011     4011
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.86% <ø> (ø)` | :arrow_up: |


------

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