DOC: update the pandas.Series.dt.total_seconds docstring
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [X] PR title is "DOC: update the <your-function-or-method> docstring"
- [X] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [X] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [X] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```
################################################################################
################## Docstring (pandas.Series.dt.total_seconds) ##################
################################################################################

Return total duration of each element expressed in seconds.

Return a series with the same length and index as the original,
containing the length of each element expressed in seconds.

Returns
-------
s : pandas.Series
    a series of type `float64`.

Examples
--------
>>> s = pd.Series(pd.to_timedelta(np.arange(5), unit='d'))
>>> s
0   0 days
1   1 days
2   2 days
3   3 days
4   4 days
dtype: timedelta64[ns]
>>> s.dt.total_seconds()
0         0.0
1     86400.0
2    172800.0
3    259200.0
4    345600.0
dtype: float64

################################################################################
################################## Validation ##################################
################################################################################

Errors found:
        Errors in parameters section
                Parameters {'args', 'kwargs'} not documented
        See Also section not found

```

If the validation script still gives errors, but you think there is a good reason
to deviate in this case (and there are certainly such cases), please state this
explicitly.


Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
