Documentation

Documentation only; no code or data changes.

• The DeviceKit migration guide shows how to check for specific models with a set of identifiers instead of enum cases.
• The overview leads with Device being Sendable: it never touches UIDevice, so it works from any thread or actor.
• Device.mac notes that an identifier Apple reused across models, such as MacBookPro15,1 for the 2018 and 2019 15-inch MacBook Pro, gives the newest of them.