Mention unsafe loading

**Your Godot version:**

4.0 stable

**Issue description:**
I watched this video https://www.youtube.com/watch?v=j7p7cGj20jU and it said they talked to the devs but I don't see the docs mention malicious code anywhere. I think it should be mentioned on the following pages if people want to use it to save and load.

**URL to the documentation page (if already existing):**

https://docs.godotengine.org/en/stable/tutorials/io/saving_games.html
https://docs.godotengine.org/en/stable/classes/class_configfile.html#class-configfile-method-save
https://docs.godotengine.org/en/stable/classes/class_resourcesaver.html#class-resourcesaver-method-save
https://docs.godotengine.org/en/stable/classes/class_fileaccess.html#class-fileaccess-method-store-var