DOC: Improved the docstring of Series.any()
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [x] PR title is "DOC: Improved the docstring of Series.any()"
- [ ] The validation script passes: `scripts/validate_docstrings.py Series.any()`
- [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 Series.any()`
- [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.any)  ########################
################################################################################

Return whether any element is True over requested axis.

Returns True if one (or more) elements are non-zero,
not-empty or not-False.

Parameters
----------
axis : {index (0)}
skipna : boolean, default True
    Exclude NA/null values. If an entire row/column is NA, the result
    will be NA.
level : int or level name, default None
    If the axis is a MultiIndex (hierarchical), count along a
    particular level, collapsing into a scalar.
bool_only : boolean, default None
    Include only boolean columns. If None, will attempt to use everything,
    then use only boolean data. Not implemented for Series.
**kwargs : Additional keywords have no effect but might be accepted for
    compatibility with numpy.

Returns
-------
any : scalar or Series (if level specified)

Examples
--------
>>> s1 = pd.Series([1, 2, 3])
>>> s1.any()
True

>>> import numpy as np
>>> s2 = pd.Series([np.NaN, np.NaN, np.NaN])
>>> s2.any()
False

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

Errors found:
        Errors in parameters section
                Parameters {'kwargs'} not documented
                Unknown parameters {'**kwargs'}
                Parameter "axis" has no description
                Parameter "**kwargs" description should start with capital letter
        See Also section not found
```

Participants of the documentation sprint were told to go ahead with the description of the "**kwargs" parameter. 

Parameter "axis" has no description looks like a bug to me. Noticed some other methods show the same validation error. e.g "pandas.Series.mean" throws the same validation error.