Rollup merge of #40838 - lukaramu:std-net-docs, r=GuillaumeGomez

Improve std::net docs

Fixes #29363

Summary:
* Added a _lot_ of missing links, both to other types/methods and to IETF RFCs, and changed occurences of just "RFC" to "IETF RFC"
* Expanded a bunch of top-level docs, specifically the module docs & the docs for `TcpListener`, `TcpStream`, `UdpSocket`, `IpAddr`, `Ipv4Addr`, `Ipv6Addr`, `SocketAddr`, `SocketAddrV4`, `SocketAddrV6`,
* Expanded method docs for `SocketAddrV6`, `AddrParseError`,
* Various edits for clarity, consistency, and accuracy

See the commit descriptions for more detail.

Things not done quite as laid out in the task list in #29363:
* `AddrParseError` still doesn't have examples, but I wasn't quite sure how to do them; other `FromStr` error types don't have any, either
* I didn't link to an IETF RFC in `IpAddr`, but in `Ipv4Addr` and `Ipv6Addr` and linked tho those from `IpAddr`; this seems more appropriate to me
* Similarly, I didn't really exand `SocketAddr`'s docs, but elaborated on `SocketAddrV4` and `SocketAddrV6`'s and linked to them from `SocketAddr`

Theres definitely still room for improvement, but this should be a good first effort :smile: