DOC: fix some doc errors/warnings
- `pd.read_csv('tmp2.sv', sep=None)` was given the warning "_ParserWarning: Falling back to the 'python' engine because the 'c' engine does not support sep=None with delim_whitespace=False; you can avoid this warning by specifying engine='python'_"  
  Is this correct? (assuming so I just added what the warning message was saying)
- with the ordered -> unordered change in the Categoricals, the example on comparing them was failing (see http://pandas-docs.github.io/pandas-docs-travis/categorical.html#comparisons)
