Fix pandas.Timedelta range
- [x] closes #12727
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

_Problem_
Pandas Timedelta derives from `datetime.timedelta` and increase
the resolution of the timedeltas. As such the Pandas.Timedelta
object can only have a smaller range of values.

_Solution_
This change modifies the properties that report
the range and resolution to reflect Pandas capabilities.

**Reference**
https://github.com/python/cpython/blob/8d1d7e6816753248768e4cc1c0370221814e9cf1/Lib/datetime.py#L651-L654

This concerns the 2nd commit, in this PR

@jreback, can you confirm that some of the tests [at](https://github.com/pydata/pandas/blob/b6f0292cd17b743528390a9e9efe92db96f8c19e/pandas/tseries/tests/test_timedeltas.py#L1678) rely on buggy output.

Specifically, that `result` (slicing in the snippet below) erroneously includes the end day.

``` python
rng = timedelta_range('1 day 10:11:12', freq='h', periods=500)
s = Series(np.arange(len(rng)), index=rng)

result = s['5 day':'6 day']
expected = s.iloc[86:110]
assert_series_equal(result, expected)
```

If this is a second bug fix, then I will address the changes documentation. Maybe there is even a better way other than peaking at the indices and creating the `expected` slices.
 pls add a whatsnew note, point to the new docs (with a `:ref:`). Pls also move the `datetime` limits section to `timeseries.rst` as well.

thanks.
 thanks @has2k1 
