Update device documentation

Sources/SwiftKit/Device/DeviceIdentifier.swift

@@ -14,6 +14,9 @@ import Foundation
  */
 public protocol DeviceIdentifier: AnyObject {
     
+    /**
+     Get a unique device identifier.
+     */
     func getDeviceIdentifier() -> String
 }
 

Sources/SwiftKit/Device/KeychainBasedDeviceIdentifier.swift

@@ -29,6 +29,13 @@ public class KeychainBasedDeviceIdentifier: DeviceIdentifier {
     private let backupIdentifier: DeviceIdentifier
     private let keychainService: KeychainService
     
+    /**
+     Get a unique device identifier from the device keychain.
+     
+     If no identifier exists in the keychain, the identifier
+     will use the provided `backupIdentifier` to generate an
+     identifier, then persist that id in the device keychain.
+     */
     public func getDeviceIdentifier() -> String {
         if let id = keychainService.string(for: key, with: nil) { return id }
         let id = backupIdentifier.getDeviceIdentifier()

Sources/SwiftKit/Device/UserDefaultsBasedDeviceIdentifier.swift

@@ -28,6 +28,13 @@ public class UserDefaultsBasedDeviceIdentifier: DeviceIdentifier {
     
     private let defaults: UserDefaults
     
+    /**
+     Get a unique device identifier from the user defaults.
+     
+     If no persisted identifier exists, this identifier will
+     generate a new identifier, then persist and return that
+     identifier.
+     */
     public func getDeviceIdentifier() -> String {
         if let id = defaults.string(forKey: key) { return id }
         return generateDeviceIdentifier()