Updated docstring for copy() method
- [x] closes #12663 

Added note that a copy with `deep=True` does not copy the data of objects contained in underlying arrays with `dtype=object`.

Actually I think `deep=False` doesn't copy the data at all:

```
d = pd.DataFrame(rand(3,3))
c = d.copy(deep=False)
c.loc[0,0] = -9
print d
```

Seems like `deep=False` is a view of the data and uses the exact same objects for index and columns,
whereas `deep=True` is a copy of the data and a copy of the index and columns (except for dtype=object)
 I tried to be more explicit in this version. I hope this is helpful.
 thanks @agartland 
