DOC/BUG: errors in the categorical docs
After #7917 still some errors in the categorical docs:
- Docs say "Writing data to a HDF store works", but `df.to_hdf(hdf_file, "frame")` gives a `TypeError: objects of type ``Categorical`` are not supported in this context, sorry` (http://pandas-docs.github.io/pandas-docs-travis/categorical.html#getting-data-in-out). Also `pd.read_hdf(hdf_file, "frame")` gives an error.
- In the section about indexing, `df.ix["h":"j",0:1]` gives a `AttributeError: 'DataFrame' object has no attribute '_is_view'` (http://pandas-docs.github.io/pandas-docs-travis/categorical.html#getting)
