DOC: documentation rendering issue for functions using typing overloads
#### Location of the documentation

https://pandas.pydata.org/docs/dev/reference/api/pandas.concat.html


#### Documentation problem
This is how the method header looks for concat:

```python
pandas.concat(objs: Union[Iterable[‘DataFrame’], Mapping[Label, ‘DataFrame’]], axis='0', join: str = "'outer'", 
ignore_index: bool = 'False', keys='None', levels='None', names='None', verify_integrity: bool = 'False', 
sort: bool = 'False', copy: bool = 'True') → ’DataFrame’
pandas.concat(objs: Union[Iterable[FrameOrSeries], Mapping[Label, FrameOrSeries]], axis='0', join: str = "'outer'", 
ignore_index: bool = 'False', keys='None', levels='None', names='None', verify_integrity: bool = 'False', 
sort: bool = 'False', copy: bool = 'True') → FrameOrSeriesUnion
```

For comparison, this is merge:
```python
pandas.merge(left, right, how='inner', on=None, left_on=None, right_on=None, left_index=False, 
right_index=False, sort=False, suffixes='_x', '_y', copy=True, indicator=False, validate=None)
```

As can be seen above, concat keeps the type information and there are two entries (likely due to the different output types).

I'm not sure why this occurs, but there are several overloaded methods in the code which could be related: https://github.com/pandas-dev/pandas/blob/54fa3da2e0b65b733d12de6cbecbf2d8afef4e9f/pandas/core/reshape/concat.py#L40-L83