chore(react): Add comment about createContext defaultValue (#39389)

types/react/index.d.ts

@@ -338,6 +338,8 @@ declare namespace React {
         displayName?: string;
     }
     function createContext<T>(
+        // If you thought this should be optional, see
+        // https://github.com/DefinitelyTyped/DefinitelyTyped/pull/24509#issuecomment-382213106
         defaultValue: T,
         calculateChangedBits?: (prev: T, next: T) => number
     ): Context<T>;