Reword the warnings about ServiceWorker mode

[mossroy]

So far, it was unstable so we stated serious warnings to discourage its use, except for testing.
Now it's a bit different : the ServiceWorker mode is more stable.
But there are still a few issues. At least :

• recent wikimedia ZIM files, when viewed on small screens, have the sections collapsed, with no possibility to open them
• when switching between jQuery and ServiceWorker modes, I still manage to break the UI. Workaround is to reload the page or app. I've created Fix broken ServiceWorker when switching between modes. #423 for that