Documentation: explain notification configuration

[mdeweerd]

It would be useful to explain what the impact of the "Foreground Service" / "Warning" configuration is.

Regarding warnings: I had serveral notifications about disconnects but they surely rapidly restored. Maybe there could be an option to have less of these.

I always have this:

And when checking the configuration:

[p1gp1g]

The foreground notification is necessary for foreground services on Android.

The warning is useful to know when your server is unreachable. It also spawn when your phone can't reach your server because of connectivity issue (if there network must still be available)