[Doc] PeerOptions missing field documentation

## What is the problem your feature solves, or the need it fulfills?

The [PeerOptions](https://docs.rs/pingora/latest/pingora/upstreams/peer/struct.PeerOptions.html) struct defers documentation to the [Peer](https://docs.rs/pingora/latest/pingora/upstreams/peer/trait.Peer.html) trait. The fields of the struct do not match that of the methods of the trait 1:1

Most notably, I am finding that the lack of documentation on idle timeouts to be a bit confusing. I have set the `idle_timeout` to 5 seconds nothing happens after making a connection. No timeout occurs.

If I set `read_timeout` to 5 seconds, after 5 seconds there is a timeout as anticipated.

Request: document `PeerOptions` fields. Notably, document the fields that do not have a corresponding value in the Peer trait.

Fields with no corresponding documentation in trait methods:

- read_timeout
- write_timeout
- no_header_eos
- max_h2_streams
- extra_proxy_headers
- curves
- second_keyshare