DOC: update the pandas.Series.str.rpartition() docstring (Delhi)
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.str.rpartition) ###################
################################################################################

Split the string at the last occurrence of `sep`.

This method splits the string at the last occurrence of `sep`,
and return 3 elements containing the part before the separator,
the separator itself, and the part after the separator.
If the separator is not found, return 3 elements containing two empty strings, followed by the string itself.

Parameters
----------
pat : string, default whitespace
    String to split on.
expand : bool, default True
    If True, return DataFrame/MultiIndex expanding dimensionality.
    If False, return Series/Index.

Returns
-------
split : DataFrame/MultiIndex or Series/Index of objects

See Also
--------
partition : Split the string at the first occurrence of `sep`

Examples
--------

>>> s = pd.Series(['A_B_C', 'D_E_F', 'X'])
>>> s
0    A_B_C
1    D_E_F
2        X
dtype: object

>>> s.str.partition('_')
   0  1    2
0  A  _  B_C
1  D  _  E_F
2  X

>>> s.str.rpartition('_')
     0  1  2
0  A_B  _  C
1  D_E  _  F
2          X

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

Docstring for "pandas.Series.str.rpartition" correct. :)

```

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.

