文档只是一句话: Sets the number and size of the buffers used for reading a response from the proxied server, for a single connection. By default, the buffer size is equal to one memory page. This is either 4K or 8K, depending on a platform.
If proxy_pass is specified without a URI, the request URI is passed to the server in the same form as sent by a client when the original request is processed, or the full normalized request URI is passed when processing the changed URI: