[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.