docs: document Linux kernel-timestamp preference for OWD

On Linux, one-way delay measurement best-effort prefers kernel
send/receive timestamps over userspace timing, falling back
whenever a kernel reading is unavailable or implausible. RTT is
unaffected and remains userspace-timed.

Add a user-facing README note near the measurement/output
explanation, and bring OneWayDelaySample::client_to_server's
rustdoc in line with the send-endpoint behavior, matching the
style already used for server_to_client. Also fix a stale
socket_io module doc that still claimed the captured kernel
receive timestamp went unconsumed; ServerCore has used it via
preferred_receive_wall_ns since d3f93a9.

crates/irtt-server/src/socket_io.rs

@@ -32,11 +32,11 @@
 //! instant, is served normally with no timestamp. Nothing about which datagrams
 //! reach [`ServerCore`](crate::ServerCore) depends on it.
 //!
-//! **Nothing consumes it yet.** It is captured here so a follow-up change can
-//! decide how a kernel-observed arrival time enters the protocol core; today
-//! the core still samples its own receive instant and no protocol timing is
-//! affected. It is a software wall-clock reading only — not a NIC hardware
-//! timestamp, not monotonic, and not a transmit timestamp.
+//! [`ServerCore`](crate::ServerCore) prefers it over its own receive sample
+//! when reporting a request's receive instant, falling back to that sample
+//! when no plausible kernel reading is available; see
+//! `preferred_receive_wall_ns`. It is a software wall-clock reading only —
+//! not a NIC hardware timestamp, not monotonic, and not a transmit timestamp.
 
 use std::{io, net::SocketAddr, time::SystemTime};