DOC: DatetimeArray.std
`DatetimeIndex.std` is missing a doc-string. Add doc-string to `DatetimeArray.std` as `DatetimeIndex` is re-using `DatetimeArray.std`.


There are a few more keywords, but they seem to be not implemented (numpy-compatiblity?)

```py
pd.DatetimeIndex([1, 2]).std(out=[])
ValueError: the 'out' parameter is not supported in the pandas implementation of std()

pd.DatetimeIndex([1, 2]).std(keepdims=True)
ValueError: the 'keepdims' parameter is not supported in the pandas implementation of std()

pd.DatetimeIndex([1, 2]).std(dtype=np.dtype("datetime64[ns]"))
ValueError: the 'dtype' parameter is not supported in the pandas implementation of std()
```

For some reason, this doc-string is needed for #43828.