iOS 11 已经修复了这个问题，此问题仅针对 iOS 10 及其低于 iOS 10 的版本！
iOS 10 中，当设置 tableViewCell 的样式为 .subtitle 时，即使设置 detailTextLabel?.numberOfLines = 0，也会出现多行文字溢出的问题，如图：

解决方法：
在 tableViewCell 中重写
systemLayoutSizeFitting(_ targetSize: CGSize, withHorizontalFittingPriority horizontalFittingPriority: UILayoutPriority, verticalFittingPriority: UILayoutPriority)
来修复此问题：

    override func systemLayoutSizeFitting(_ targetSize: CGSize, withHorizontalFittingPriority horizontalFittingPriority: UILayoutPriority, verticalFittingPriority: UILayoutPriority) -> CGSize {
        
        if #available(iOS 11, *) {
            return super.systemLayoutSizeFitting(targetSize, withHorizontalFittingPriority: horizontalFittingPriority, verticalFittingPriority: verticalFittingPriority)
        }
        else {
            layoutIfNeeded()
            
            var size = super.systemLayoutSizeFitting(targetSize, withHorizontalFittingPriority: horizontalFittingPriority, verticalFittingPriority: verticalFittingPriority)
            
            guard let textLabel = textLabel,
                  let detailTextLabel = detailTextLabel
            else { return size }
            
            let detailHeight = detailTextLabel.frame.height
            if detailTextLabel.frame.minX > textLabel.frame.minX {
                let textHeight = textLabel.frame.height
                
                if detailHeight > textHeight {
                    size.height += detailHeight - textHeight
                }
            }
            else {
                size.height += detailHeight
            }
            
            return size
        }
    }