Update docs for resolver.unstable_conditionNames to include default s…

…et by RN CLI

Summary: See react-native-community/cli#1862.

Reviewed By: jacdebug

Differential Revision: D43876358

fbshipit-source-id: 0b075956ef8e6254beed7c8572b1c99ba9649ffe

docs/Configuration.md

@@ -342,6 +342,12 @@ Conditions may be any string value and are resolved in the order specified by ea
 
 Defaults to `['import', 'require']`.
 
+:::note
+
+When Metro is started via React Native CLI, `conditionNames` defaults to `['import', 'require', 'react-native']`.
+
+:::
+
 #### `unstable_conditionsByPlatform` <div class="label experimental">Experimental</div>
 
 Type: `{[platform: string]: Array<string>}`