docs(webpack): improve typescript usage

as discussed in styleguidist/react-docgen-typescript#71 (comment), some compiler options break component discovery.  the safest strategy is to always include the tsconfig from user space, which improves component detection.

docs/Webpack.md

@@ -97,7 +97,7 @@ If you’re using [Create React App](https://github.com/facebook/create-react-ap
 
    ```javascript
    module.exports = {
-     propsParser: require('react-docgen-typescript').parse
+     propsParser: require('react-docgen-typescript').withCustomConfig('./tsconfig.json').parse
    }
    ```