Discourage using webpack loader for all the files (#1452)
Discourage using webpack loader for all the files (#1452)

* Replaces the phrase "It is safe to enable this loader for all the files" with "It is _possible_ to enable...", because it's neither safe (as proven in #1449) nor usually necessary.
* Switches first example of Webpack config (suboptimal, applying loader to all the files) with the second one (optimal, applying loader to react-dom only, if you're using babel plugin)
gaearon/react-hot-loader,gaearon/react-hot-loader