DOC/BLD: attributes initialized as None not picked up by autosummary anymore
I already mentioned it here: https://github.com/pydata/pandas/issues/5899#issuecomment-32967854, but then this did not appear on the dev doc builds on the website and only locally on my computer. But now I also see it there. See eg http://pandas.pydata.org/pandas-docs/dev/generated/pandas.DataFrame.html#pandas.DataFrame in the list of attributes, the `is_copy` is no link anymore.

So I get the following warning: `DataFrame.is_copy.rst:: WARNING: document isn't included in any toctree` for 8 methods:

```
pandas.DataFrame.is_copy
pandas.DatetimeIndex.name
pandas.DatetimeIndex.offset
pandas.Index.asi8
pandas.Index.name
pandas.Panel.is_copy
pandas.Panel4D.is_copy
pandas.Series.is_copy
```

Strange thing is, there have not been recent changes to these functions (I think), but the errors only appeared recently. 
@y-p You did the most recent doc build on the website? (does this not happen automatically anymore?)
