TODO: document when to check the system and save paths

[markwkidd]

1. clarify that API does not support changing the system or save paths once the core is running
2. clarify when the core should check the system and save paths -- retro_load_game so that the core can fall back on the content path if the frontend has not set system or save path
3. clarify that the core should fall back on the content path, if indeed that is the standard behavior