Document missing PeerOptions fields

[ting-hong-shieh]

Summary

• document read_timeout and write_timeout as per-operation HTTP I/O limits rather than total request limits
• clarify that idle_timeout starts only after a reusable connection returns to the pool and does not govern active sessions
• document the HTTP/2 stream limit, extra CONNECT proxy headers, TLS group list, and post-quantum second-key-share option

Testing

• cargo fmt --all -- --check
• cargo doc -p pingora-core --no-deps with Rust 1.97.1
• cargo +1.85.0 doc -p pingora-core --no-deps --offline
• verified the new text in the rendered PeerOptions rustdoc page

Both rustdoc runs report one existing private-intra-doc-link warning in protocols/http/v2/server.rs; this change adds no warnings.

Closes #302