DOC: Fix docstring of window.Expanding/Rolling.apply and document *args and **kwargs
This pull request will 

- Fix docstring formatting for ```pandas.core.window.Expanding.apply``` and ```pandas.core.window.Rolling.apply```

- Document ```*args``` and ```**kwargs``` parameters:
The following functions had ```*args``` and ```**kwargs``` added to their docstring:
```Rolling.max``` 
```Expanding.max```  
```EWM.mean``` 
```EWM.std``` 
```EWM.var``` 
The following functions had ```**kwargs``` added to their docstring:
```Rolling.skew```                          
```Expanding.skew```                        
```Rolling.cov``` 
```Expanding.cov```  
```EWM.cov``` 
```EWM.corr```

