[safariservices] Updates (some deprecations) for beta 1 (#4206)

src/SafariServices/SSEnums.cs

@@ -55,6 +55,7 @@ namespace SafariServices {
 	[iOS (11,0)]
 	[Native]
 	[ErrorDomain ("SFAuthenticationErrorDomain")]
+	[Deprecated (PlatformName.iOS, 12,0, message: "Use 'ASWebAuthenticationSessionErrorCode' instead.")]
 	public enum SFAuthenticationError : long {
 		CanceledLogin = 1,
 	}

src/safariservices.cs

@@ -152,6 +152,7 @@ namespace SafariServices {
 	[iOS (11,0)]
 	[BaseType (typeof (NSObject))]
 	[DisableDefaultCtor]
+	[Deprecated (PlatformName.iOS, 12,0, message: "Use 'ASWebAuthenticationSession' instead.")]
 	interface SFAuthenticationSession {
 		[Export ("initWithURL:callbackURLScheme:completionHandler:")]
 		IntPtr Constructor (NSUrl url, [NullAllowed] string callbackUrlScheme, SFAuthenticationCompletionHandler completionHandler);