DOC: Clarified wording of coerce_float
I think this is the intended meaning (and it matches with use).
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15164?src=pr) is 85.54% (diff: 100%)
> Merging [#15164](https://codecov.io/gh/pandas-dev/pandas/pull/15164?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

```diff
@@             master     #15164   diff @@
==========================================
  Files           145        145          
  Lines         51404      51404          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43976      43976          
  Misses         7428       7428          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [684c4d5...a886538](https://codecov.io/gh/pandas-dev/pandas/compare/684c4d5f5c81e90d3cf07d1c5f1bc7c98266333a...a886538883e87f6d48547c161042ec256725d78f?src=pr) thanks!