AS guide: first version of a section on output safety, trying to give some context, but trying to go not too far away from the topic of the guide which is AS-only

AS guide: Object#extend_with_included_modules_from has been removed in ccec730d

AS guide: documents the file where each method is defined to allow cherry-picking (covers up to Hash)

AS guide: documents the file where each method is defined to allow cherry-picking (covers up to String)