DOC: mention that stack/unstack implicicly sort
It took me a while to discover what was causing this TypeError... I guess this can be useful to other people.

(Yes, I know there is a note somewhere warning users that some operations depend on the index being sorted, but maybe this is a sligthly different case, since the operations do work after all, the problem is they do something more than what is documented)

Oops, done... by the way: maybe this info could go also/instead in the docs for the two methods, I'm not sure.
 Yes, I think you can add a note in the docstring.
 thanks!
