Docs: MultiIndex support is hardly bleeding edge, remove docs warnings.
Merge pull request #7767 from lexual/remove_multi_index_bleeding_edge_comment

Docs: MultiIndex support is hardly bleeding edge, remove docs warnings.
Closes #7758 - astype(unicode) returning unicode.

Just calls numpy.unicode on all the values.
Seems to work alright on python2 and python3.

Added bugfix of #7758 to v0.15.0 changelog.