Summary:
Added language to explain that you still need to add keys for each section even if you use a keyExtractor method.
No code changed; just clarified documentation.
Closes https://github.com/facebook/react-native/pull/15007
Differential Revision: D5425897
Pulled By: hramos
fbshipit-source-id: db44064a28a673feeda5a6765ea45217d3ae51e2