Update "Game saves compatibility", mention upgrading saves api

I wrote this a while ago https://adventuregamestudio.github.io/ags-manual/GameSavesCompatibility.html
in 3.6.2 there's now a script api that lets to upgrade saves (to some degree). It's explained here:
https://adventuregamestudio.github.io/ags-manual/UpgradeTo362.html#loading-old-saves-feature
but I forgot to also explains this in the above topic.