Suspend then hibernate should be a "battery only" feature(?)

[ViBE-HU]

This is just a recommendation. But here's why:

First of all we have now plenty features via your plugin. It is an arsenal. But even well informed users do mistakes sometimes. And losing data can be painful. The problem with this function is that this is not a reversed hybrid sleep/suspend or something like that. It is designed to rely on battery presence. Even if a system has no battery the suspend state will going to work which can misleading for inexperienced desktop users. But it cannot save from data loss like hybrid sleep do if blackout happens.

How suspend then hibernate works:

1. Suspends the system
2. Then waits till the battery charge reaches the critical level
3. Hibernates the system when the critical level reached, otherwise it does nothing just keep suspended

But desktop machines never reaches the critical level. So they stay only suspended which is pretty unfortunate if a random blackout happens. Of course the exact state can be opted out in the settings but it will remains as a risk for those who don't know the difference between these states and/or not as experienced.

The related documentation:
https://www.freedesktop.org/software/systemd/man/latest/systemd-sleep.conf.html#suspend-then-hibernate