docs: Add doc for http config

[dentiny]

Rationale for this change

I'm using opendal to do large blob transfer (50GBps network, 20-30 GBps cluster-wise throughput), and found current HTTP optimization miss some explanations.

What changes are included in this PR?

Add some usage pattern I did in our production environment:

• hyper-util currently always reuse the TCP connection when HTTP/2, which leads to bottleneck on single TCP connections
  • To address, we could either maintain a reqwest client pool with multiple HTTP/2 connection underlying and round-robin; or actively act gating on max allowed concurrency so we don't create excessive connections
  • For my use case, I observe 1800 connections created on single host, if no limit applied, which is definitely bad for performance and reliability

Are there any user-facing changes?

No

AI Usage Statement

I update the doc myself, no AI involvement.

[Xuanwo]

Thank you!