Update sectionList keyExtractor docs

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

Libraries/Lists/SectionList.js

@@ -129,7 +129,8 @@ type OptionalProps<SectionT: SectionBase<any>> = {
   /**
    * Used to extract a unique key for a given item at the specified index. Key is used for caching
    * and as the react key to track item re-ordering. The default extractor checks item.key, then
-   * falls back to using the index, like react does.
+   * falls back to using the index, like react does. Note that this sets keys for each item, but 
+   * each overall section still needs its own key.
    */
   keyExtractor: (item: Item, index: number) => string,
   /**