document is_copy
#### Code Sample, a copy-pastable example if possible

#### Problem description

I find the behavior of ``SettingWithCopyWarning`` quite surprising, but I guess that's just what it is.
It would be great if you could document ``is_copy`` and how to use it, though.

Whenever any function returns a dataframe, it seems like it should make sure that ``is_copy`` is set to ``False`` (or ``None``?) so the user doesn't get a warning if they change it - if you're returning a dataframe, it's unlikely that the user expects this to be a view, and you're not doing chained assignments.

The ``is_copy`` attribute has an empty docstring in the docs and I couldn't find any explanation of it on the website (via google). The only think that told me that overwriting this attribute is actually the right thing to do (again, which is pretty weird to me), was https://github.com/pandas-dev/pandas/issues/6025#issuecomment-32904245
