date_range() option for closed='both' documented but not implemented
#### Problem description

The docstring for `pandas.date_range()`'s `closed` argument says:     
```
closed : {None, 'left', 'right'}, optional
        Make the interval closed with respect to the given frequency to
        the 'left', 'right', or both sides (None, the default).
```
Basically, this argument sets whether the date range includes or excludes the start and end dates. But I think 'both' is not possible. 

Either the docstring needs changing or (preferably) a 'both' option should be added.

#### Links to the relevant code

Here's the `date_range()` function (whose docstring I copied above):
https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/datetimes.py#L1403
it passes `closed` to `DatetimeArray._generate_range()`

which is here:
https://github.com/pandas-dev/pandas/blob/master/pandas/core/arrays/datetimes.py#L421
and which passes `closed` to: `dtl.validate_endpoints(closed)`

which is here, and is where the `validate_endpoints(closed)` would need changing to add a 'both' option, or to allow 'left' and 'right' to be set simultaneously:
https://github.com/pandas-dev/pandas/blob/master/pandas/core/arrays/datetimelike.py#L1471-1501 

Thanks!
