DOC: remove mention of TimeSeries in docs
As a Series with a DatetimeIndex is no longer presented as a `TimeSeries`, I think we also should not longer mention it in the docs as a 'seperate object', so removed the last few mentions.

maybe add this to the Deprecatoon list on 0.16 (the aliasing of the TimeSeries object itself)
 Do you mean #6581?
 yep
 actually this needs to be deprecated first, so pls create an issue - can do it in 0.15
 @jorisvandenbossche moving to 0.16.0 but ok if you wanto do in 0.15.0
 @jorisvandenbossche ?
 Looking back at this, I don't really see a problem with already changing the docs, before actually deprecating/removing it. As normal users won't see it anymore (a Series with DatetimeIndex does not give you a TimeSeries back, but just a Series).

Or what was the reason we didn't merge this?
 @jorisvandenbossche I think this is fine. the deprecation issue (for actual `TimeSeries` object is already noted)
