IntervalArray not documented in What's New for 0.24.0rc1
#### Problem description

The new type `IntervalArray` is in the API reference, but is not mentioned in What's New in the docs for v0.24.0rc1 .  The existence of the new type is probably something people would like to know about.

http://pandas.pydata.org/pandas-docs/version/0.24.0rc1/api/arrays.html#interval-data has it listed there, but in http://pandas.pydata.org/pandas-docs/version/0.24.0rc1/whatsnew/v0.24.0.html it appears in an example, and that there is a new method for the type.

Not sure if @TomAugspurger or @jschendel should look at that.