DOC: Add documentation for freq='infer' option of DatetimeIndex and TimedeltaIndex constructors
There currently doesn't appear to be any information regarding passing `freq='infer'` to the `DatetimeIndex` or `TimedeltaIndex` constructors; I only became aware of this option from reading over the code.  Would be nice to document this feature in the docstrings for `DatetimeIndex` and `TimedeltaIndex`.  Possibly in `timeseries.rst` and `timdeltas.rst` too. 

Essentially, this allows users to set the frequency of the index as the inferred frequency upon creation:
```python
In [2]: pd.DatetimeIndex(['2018-01-01', '2018-01-03', '2018-01-05'], freq='infer')
Out[2]: DatetimeIndex(['2018-01-01', '2018-01-03', '2018-01-05'], dtype='datetime64[ns]', freq='2D')
```

Note that the frequency is not automatically inferred if nothing is passed to the `freq` parameter:
```python
In [3]: pd.DatetimeIndex(['2018-01-01', '2018-01-03', '2018-01-05'])
Out[3]: DatetimeIndex(['2018-01-01', '2018-01-03', '2018-01-05'], dtype='datetime64[ns]', freq=None)
```

Similar example with `TimedeltaIndex`:
```python
In [4]: pd.TimedeltaIndex(['0 days', '10 days', '20 days'], freq='infer')
Out[4]: TimedeltaIndex(['0 days', '10 days', '20 days'], dtype='timedelta64[ns]', freq='10D')

In [5]: pd.TimedeltaIndex(['0 days', '10 days', '20 days'])
Out[5]: TimedeltaIndex(['0 days', '10 days', '20 days'], dtype='timedelta64[ns]', freq=None)
```