DOC: Improved the docstring of pandas.DataFrame.values
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

```
################################################################################
############ Docstring (pandas.pandas.core.generic.NDFrame.values)  ############
################################################################################

Generate and return a Numpy representation of NDFrame.

Only the values in the NDFrame will be returned, the axes labels will
be removed.

Returns
-------
numpy.ndarray
    The values of the NDFrame

Examples
--------
>>> df = pd.DataFrame([('falcon', 'bird',    389.0),
...                    ('parrot', 'bird',     24.0),
...                    ('lion',   'mammal',   80.5),
...                    ('monkey', 'mammal', np.nan)],
...                   columns=('name', 'class', 'max_speed'))
>>> df
     name   class  max_speed
0  falcon    bird      389.0
1  parrot    bird       24.0
2    lion  mammal       80.5
3  monkey  mammal        NaN
>>> df.values
array([['falcon', 'bird', 389.0],
       ['parrot', 'bird', 24.0],
       ['lion', 'mammal', 80.5],
       ['monkey', 'mammal', nan]], dtype=object)

Notes
-----
The dtype will be a lower-common-denominator dtype (implicit
upcasting); that is to say if the dtypes (even of numeric types)
are mixed, the one that accommodates all will be chosen. Use this
with care if you are not dealing with the blocks.

e.g. If the dtypes are float16 and float32, dtype will be upcast to
float32.  If dtypes are int32 and uint8, dtype will be upcast to
int32. By numpy.find_common_type convention, mixing int64 and uint64
will result in a flot64 dtype.

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

Errors found:
        Private classes (['NDFrame']) should not be mentioned in public docstring.
        See Also section not found
```