Update HMR docs

[alexeyr-ci]

E.g. we have

react_on_rails/docs/guides/hmr-and-hot-reloading-with-the-webpack-dev-server.md

Line 34 in 846d02d

`dev_server.inline` maps to [devServer.inline](https://webpack.js.org/configuration/dev-server/#devserverinline).

but devServer.inline doesn't seem to exist and the option may have been removed on our side too.