Update sectionlist.md for keyExtractor (facebook#4341)

keyExtractor in sectionlist checks for item.key first. If it is not present it checks for item.id. If item.id is not present then it falls back to index.

Author: YashParekh1301

docs/sectionlist.md

@@ -179,7 +179,7 @@ Rendered in between each item, but not at the top or bottom. By default, `highli
 
 ### `keyExtractor`
 
-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. Note that this sets keys for each item, but each overall section still needs its own key.
+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 `item.id`, and then 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.
 
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