DOC: Mention that boolean indexing is impossible for new nullable integer/boolean and string data types when they contain missing values
The new data types nullable integer, nullable boolean, and string cannot do boolean indexing when the columns contain missing values. 

I searched the [pd.NA][0] and the [boolean indexing][1] sections and have not seen this mentioned. Users need to be aware of this limitation as it greatly reduces their usability.

[0]: https://pandas.pydata.org/docs/user_guide/missing_data.html#experimental-na-scalar-to-denote-missing-values
[1]: https://pandas.pydata.org/docs/user_guide/indexing.html#boolean-indexing