Document macOS URLTransfer uses out of date transfer type name

Part of #2126

Author: jonahgraham

bundles/org.eclipse.swt/Eclipse SWT Drag and Drop/cocoa/org/eclipse/swt/dnd/URLTransfer.java

@@ -33,6 +33,8 @@
 public class URLTransfer extends ByteArrayTransfer {
 
 	static URLTransfer _instance = new URLTransfer();
+	// XXX: SWT uses the deprecated format name here,
+	// the new name is "public.url" aka NSPasteboardTypeURL
 	static final String URL = OS.NSURLPboardType.getString();
 	static final String URL1 = OS.kUTTypeURL.getString();
 	static final int URL_ID = registerType(URL);