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.
