DOC: index described as a "set" but permits duplicates -- change to multiset or list?
#### Location of the documentation

https://pandas.pydata.org/pandas-docs/stable/reference/indexing.html


#### Documentation problem

Index is described as a "set", but behaves as a multiset: len() counts each occurrent of a duplicate element separately.

Also, Index is described as an "ordered set", but that term connotes a set with an order relation on the _values_ of its elements.  

#### Suggested fix for documentation

An index seems more like an immutable ordered _list_; maybe, describe it as that?  