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)
the spaces before `removed` should be removed