Update DataFrame.to_feather() docstring/api documentation and deprecate fname argument.
#### Code Sample, a copy-pastable example if possible

This code works. 

```python
import pandas as pd
import pandas.util.testing as tm
from io import BytesIO

df = pd.DataFrame({"a": [1, 2, 3], "b": list("abc")})

buf = BytesIO()
df.to_feather(fname=buf)
result = pd.read_feather(path=buf)

tm.assert_frame_equal(result, df)
```
#### Problem description

Documentation doesn't mention accepting a buffer.

```
    def to_feather(self, fname):
        """
        Write out the binary feather-format for DataFrames.

        Parameters
        ----------
        fname : str
            String file path.
        """
```
#### Expected Output

change argument name to better reflect types accepted and be consistent with other io methods.

would need to deprecate `fname` argument.

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]

</details>
