react: Add link about SyntheticEvent.target

react/index.d.ts

@@ -284,6 +284,7 @@ declare namespace React {
         stopPropagation(): void;
         isPropagationStopped(): boolean;
         persist(): void;
+        // If you thought this should be `EventTarget & T`, see https://github.com/DefinitelyTyped/DefinitelyTyped/pull/12239
         target: EventTarget;
         timeStamp: Date;
         type: string;