Documation: copy documenation is lacking detail for when deep=False
The DataFrame.copy documentation (for pandas 0.22.0) states:

```
deep : boolean or string, default True
Make a deep copy, including a copy of the data and the indices. 
With deep=False neither the indices or the data are copied.
Note that when deep=True data is copied, actual python objects 
will not be copied recursively, only the reference to the object. 
This is in contrast to copy.deepcopy in the Standard Library,
which recursively copies object data.
```
#### Problem description

The documentation does not state what *is* copied when ``deep=False`` (it doesn't copy data, it doesn't copy indices, what does it copy then?).

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