Document better publicPath need for code splitting

[fjsj]

We need to document better that publicPath should be used when using code splitting. Right now this is only documented as a comment in the code splitting example. We need to explain that setting publicPath to STATIC_URL is needed because that's the only case where webpack needs to know about STATIC_URL.

When people need to integrate publicPath with the S3 assets path when using Django for collectstatic, they get confused by that: https://github.com/django-webpack/django-webpack-loader/issues/287