[CR] Autosave turns clarification

Fixes #8929 

This is my attempt at fixing the issue I raised previously. Please check it, as I am new to Git and haven't done any coding for many years.

The effect should be that autosaves occur as they currently do, but the UI is clearer.
* AUTOSAVE_TURNS is now in actual turns, not tens of turns.
* the autosave code in game.cpp no longer multiplies AUTOSAVE_TURNS by 10
* The explanatory text for the AUTOSAVE option states that it is by turns or real time, whichever is later.
Yeah, I'm not really sure why this multiplication was needed.  Might've been something with the modulo function there but I've not seen that elsewhere.