ExtensionArray.astype docstring
https://github.com/pandas-dev/pandas/blob/6d3565a5181b3d0fb8342e69dd1d21a98ac3d380/pandas/core/arrays/base.py#L324

Should the docstring say that this should return an ExtensionArray when an ExtensionType is passed into it?
I note that PeriodArray behaves like this. 
```
import pandas as pd
a=pd.core.arrays.period_array([2000, 2001, 2002], freq='D')
type(a.dtype)
```

pandas.core.dtypes.dtypes.PeriodDtype

```
type(a.astype(a.dtype))
```
pandas.core.arrays.period.PeriodArray
