docs: Rationalize wsjson buffer pooling and explain tradeoffs

I see that this library is using `sync.Pool` to pool buffers for wsjon sub-package. `sync.Pool` is suitable only for pooling objects of same size. If they are of different size, you have a problem: those buffers will just grow and grow never to shrink.

See https://github.com/golang/go/issues/23199 for more background.

定位 wsjson 子包及其 sync.Pool 缓冲区实现，然后阅读关于池化不同大小对象的链接 Go issue。确定当前的缓冲区大小行为，并记录现有池化方法的权衡。池化策略得到合理化说明并且其局限性得到解释，即表示完成。

由索引模型根据 Issue 内容生成。