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.

README.md

@@ -185,6 +185,8 @@ Useful measurement fields include:
 
 Adjusted RTT can be negative when server processing exceeds the measured raw RTT. One-way delay estimates can be negative because of clock skew between the client and server.
 
+On Linux, `sd_us` and `rd_us` (and the underlying one-way delay samples) prefer kernel-captured send/receive timestamps over userspace timing where the platform and socket support them, reducing the effect of scheduling and queuing delay on these measurements. This is a best-effort fallback: unsupported or implausible kernel timestamps fall back to userspace timing. RTT always uses userspace timing and is unaffected.
+
 Default table output prints a final summary after completed finite runs and interrupted continuous runs when the run policy permits it. CSV, TSV, and JSON Lines output do not print this summary.
 
 A peer closing a finite CLI or TUI run is accepted as a terminal outcome. In