API: deprecate range-generating keywords in DatetimeIndex
I was reviewing the PR on the DatetimeIndex docstring, and the list of parameters is horribly complex because you can both use it by passing `data` (with accompanying keywords like `copy`, `dtype`, ..) and by generating a range with optional `start`, `end`, `periods`. 

Since for generating ranges, we have the special purpose `pandas.date_range`, do we need to keep this functionality in the `DatetimeIndex` constructor as well?

Of course deprecating it will annoy people who use it (although I *think* the majority of usage of DatetimeIndex passed `data`), so the question is whether it is worth it.