Respond to CR.

// FREEBIE

src/Messages/Attachments/TSAttachmentPointer.h

@@ -17,13 +17,11 @@ typedef NS_ENUM(NSUInteger, TSAttachmentPointerState) {
  */
 @interface TSAttachmentPointer : TSAttachment
 
-- (instancetype)init NS_UNAVAILABLE;
-
 - (instancetype)initWithServerId:(UInt64)serverId
                              key:(NSData *)key
                           digest:(nullable NSData *)digest
                      contentType:(NSString *)contentType
-                           relay:(NSString *)relay;
+                           relay:(NSString *)relay NS_DESIGNATED_INITIALIZER;
 
 @property (nonatomic, readonly) NSString *relay;
 @property (atomic) TSAttachmentPointerState state;