[Encore] Update docs related to HMR

[weaverryan]

Thanks to symfony/webpack-encore#8, Encore supports HMR.

But I've put this on hold, because we don't actually have any loaders yet that support HMR, so turning it on doesn't do anything.

[javiereguiluz]

Closing as fixed because HMR is explained here: https://symfony.com/doc/current/frontend/encore/dev-server.html