DOC: Fixed type hints for GroupBy.{any, all}
The current documentation says the return value of GroupBy.all and GroupBy.any is `bool`.

```
    def any(self, skipna: bool = True):
        """
        Return True if any value in the group is truthful, else False.

        Parameters
        ----------
        skipna : bool, default True
            Flag to ignore nan values during truth testing.

        Returns
        -------
        bool
        """
```

However, the actual returned type is DataFrame or Series (with the same shape as `GroupBy.sum()`).

```
In [25]: df
Out[25]:
     0    1    2    3
0  1.0  0.0  0.0  0.0
1  0.0  1.0  0.0  0.0
2  0.0  0.0  1.0  0.0
3  0.0  0.0  0.0  1.0

In [26]: df.groupby(0).any()
Out[26]:
         1      2      3
0
0.0   True   True   True
1.0  False  False  False

In [27]: type(df.groupby(0).any())
Out[27]: pandas.core.frame.DataFrame
```

This PR fixes the wrong type hints for `GroupBy.{any, all}`.

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

That is a bit short imho. Could you maybe write something like here ? https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.core.groupby.GroupBy.sum.html Okay. I wrote some drafts for `GroupBy.all()` documentation. Which one do you think better?

1. Computed all of values within each group.
    - just replaced `sum` by `all` in GroupBy.sum documentation.
2. Computed all() of values within each group.
    - added parentheses to make it clear that `all` represents somewhat function (that is, it's not a pronoun).
3. DataFrame or Series of boolean values, which is True if all elements are True within each group, False otherwise.
    - wrote whole description.

If you have any alternative, plz tell me. Hello @ytakashina! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-10-09 00:30:11 UTC Edited the description for the return value of GroupBy.{any, all} after receiving @rhshadrach's suggestion. Seems like a clear improvement over the status quo.  @rhshadrach are you happy here? @ytakashina Can you merge master? I think it will fix CI. @rhshadrach CI Fixed! Thanks @ytakashina!