DOC: Categorical sort_values and sort Documentation
In `categorical.py`, we enforce the fact that `self` must be ordered when calling `min` or `max`.  However, `self` can be unordered when calling `sort_values`.  This doesn't make sense in my mind, for if you can sort the values for unordered `self`, then I can then find a minimum value of `self`.  The same comment applies to `argsort` as well.
