-deleteItem: Explain some Mac-only code

SSKeychain/SSKeychainQuery.m

@@ -71,6 +71,15 @@ - (BOOL)deleteItem:(NSError *__autoreleasing *)error {
 #if TARGET_OS_IPHONE
 	status = SecItemDelete((__bridge CFDictionaryRef)query);
 #else
+	// On Mac OS, SecItemDelete will not delete a key created in a different
+	// app, nor in a different version of the same app.
+	//
+	// To replicate the issue, save a password, change to the code and
+	// rebuild the app, and then attempt to delete that password.
+	//
+	// This was true in OS X 10.6 and probably later versions as well.
+	//
+	// Work around it by using SecItemCopyMatching and SecKeychainItemDelete.
 	CFTypeRef result = NULL;
 	[query setObject:@YES forKey:(__bridge id)kSecReturnRef];
 	status = SecItemCopyMatching((__bridge CFDictionaryRef)query, &result);