DOC: actually document float_precision in read_csv
So I wasn't 100% correct when I said that `float_precision` was documented <a href="https://github.com/pydata/pandas/issues/12686#issuecomment-222684918">here<a/>.  It was well documented internally for `TextParser` and in a section for `io.rst`, but it wasn't listed formally in the parameters for the `read_csv` documentation.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13377?src=pr) is **84.23%**

> Merging [#13377](https://codecov.io/gh/pydata/pandas/pull/13377?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
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  Lines         50739      50739          
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  Hits          42740      42740          
  Misses         7999       7999          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [b722222...ffca1ec](https://codecov.io/gh/pydata/pandas/compare/b722222f5ea760a3f3df4d063309949eb4956674...ffca1ecd64325073365267aa1bb6c674ac42deca)
 ty
