Describe what the connected-UDP modules are instead of forecasting their wiring

Master-line half of the same correction made on the maintenance line. The header
comments on the connected-socket module inside the UDP io layer and on the
per-peer drain thread both said the fast path was not yet wired into the
encrypt-worker dispatch site, and that a follow-up would do it. It has been
wired at both ends for some time: the rx loop calls
activate_connected_udp_sessions, which spawns PeerRecvDrain. A reader deciding
whether the connected-UDP send path is live got the wrong answer from the first
comment they met.

Both modules already document themselves, so the forecast comments are deleted
rather than reworded, and the dead_code allowances they justified go with them.
The bare allowance on the connected-socket handle, which carried no comment at
all, goes too. None of the three was suppressing anything: clippy with
-D warnings passes with all of them removed, so besides being unjustified they
would have hidden whatever went dead next in those modules. The item-level
allowance on local_addr is a different attribute and is left alone.

src/peer/connected_udp/drain.rs

@@ -1,8 +1,3 @@
-// Paired with the connected-socket opener (`io::open_connected_fd`):
-// dormant in this PR until the activation handler is wired into the
-// node tick (follow-up).
-#![allow(dead_code)]
-
 //! Recv-side drain thread for a per-peer connected UDP socket.
 //!
 //! Once a UDP socket is `connect()`-ed to a peer, Linux and Darwin

src/peer/connected_udp/socket.rs

@@ -3,7 +3,6 @@
 //! Adopts an fd produced by `crate::transport::udp::open_connected_fd`
 //! and closes it on drop. See that function's docs for why established
 //! peers get their own connected socket.
-#![allow(dead_code)]
 
 use std::net::SocketAddr;
 use std::os::unix::io::{AsRawFd, OwnedFd, RawFd};

src/transport/udp/io.rs

@@ -803,11 +803,6 @@ pub use platform::{AsyncUdpSocket, UdpRawSocket};
 /// is libc-syscall + `sockopts_macos` specific.
 #[cfg(any(target_os = "linux", target_os = "macos"))]
 mod connected {
-    // The connected-UDP fast path is infra-ready but not yet wired into
-    // the encrypt-worker dispatch site (a follow-up PR will refcount-clone
-    // the socket into each FmpSendJob). Keep the API surface in tree.
-    #![allow(dead_code)]
-
     use std::io;
     use std::net::SocketAddr;
     use std::os::unix::io::{AsRawFd, FromRawFd, OwnedFd, RawFd};