Document recognized as a streaming response behavior

[miohtama]

The reverse proxy documentation refers to recognising streaming responses. However it is unspecified how this is done, making it guess work for the upstream application developers how to support Caddy when reverse responses should not be buffered.

flush_interval is a duration value that adjusts how often Caddy should flush the response buffer to the client. By default, no periodic flushing is done. A negative value disables response buffering, and flushes immediately after each write to the client. This option is ignored when the upstream's response is recognized as a streaming response, or if its content length is -1; for such responses, writes are flushed to the client immediately.