BUG/DOC: DataFrame.values return type when uint64 is mixed with signed int types
`DataFrame.values`' doc mentions type promotion but leaves out that when `uint64` is mixed with signed `int` types, the return type is `int64`, which can be surprising.  Perhaps consider returning `dtype(object)` in `internals._interleaved_dtype` when `uint64` and signed `int`s are present?
