docs(JestObjectAPI jest-mock): Add explanation for __esModule: true

Issue jestjs#8543

docs/JestObjectAPI.md

@@ -240,7 +240,7 @@ const moduleName = require('../moduleName');
 moduleName(); // Will return '42';
 ```
 
-When using the `factory` parameter for an ES6 module with a default export, the `__esModule: true` property needs to be specified:
+When using the `factory` parameter for an ES6 module with a default export, the `__esModule: true` property needs to be specified. This property is normally generated by Babel / TypeScript, but here it needs to be set manually. When importing a default export, it's an instruction to import the property named `default` from the export object:
 
 ```js
 import moduleName, {foo} from '../moduleName';