DOC: Update the pandas.Series.str.find() (Delhi)
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [x] PR title is "DOC: update the pandas.Series.str.find docstring"
- [x] The validation script passes: `scripts/validate_docstrings.py pandas.Series.str.find`
- [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 pandas.Series.str.find`
- [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.find) ######################
################################################################################

Retrieve least index of substring.

Return lowest indexes in each strings in the Series/Index
where the substring is fully contained between [start:end].
Return -1 on failure. Equivalent to standard :meth:`str.find`.

Parameters
----------
sub : str
    Substring being searched.
start : int
    Left edge index.
end : int
    Right edge index.

Returns
-------
found : Series/Index of integer values

Examples
--------
>>> s = pd.Series('Finding with pandas')
>>> s.str.find('wi')
0    8
dtype: int64

Returns -1 if the substring is not found in S

>>> s.str.find('wi', start=0, end=5)
0   -1
dtype: int64

See Also
--------
rfind : Return highest indexes in each strings

################################################################################
################################## Validation ##################################
################################################################################
```

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