docs: scope kernel-timestamp claim to the Tokio server runtime

irtt_server::ServerCore::handle_datagram is a public API that
always passes kernel_rx_timestamp = None; only the crate-private
entry point the Tokio runtime (the irtt-server applet) uses
supplies one. Say so explicitly instead of implying every use of
this crate's server gets the preference.

README.md

@@ -185,7 +185,7 @@ 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 a kernel-captured timestamp over userspace timing for this client's own send/receive endpoint where the platform and socket support it, reducing the effect of scheduling and queuing delay on these measurements. The peer's reported endpoint gets the same preference when it is this crate's own `irtt-server` on Linux; against another IRTT-compatible server, whatever wall-clock value it reports is used as-is. This is a best-effort fallback: unsupported or implausible kernel timestamps fall back to userspace timing. `raw_rtt_us` always uses userspace timing and is unaffected; `adjusted_rtt_us`/`effective_rtt_us` can still shift slightly, since server processing time is derived from the same server receive endpoint.
+On Linux, `sd_us` and `rd_us` (and the underlying one-way delay samples) prefer a kernel-captured timestamp over userspace timing for this client's own send/receive endpoint where the platform and socket support it, reducing the effect of scheduling and queuing delay on these measurements. The peer's reported endpoint gets the same preference when it is the `irtt-server` applet running on Linux; against another IRTT-compatible server, whatever wall-clock value it reports is used as-is. This is a best-effort fallback: unsupported or implausible kernel timestamps fall back to userspace timing. `raw_rtt_us` always uses userspace timing and is unaffected; `adjusted_rtt_us`/`effective_rtt_us` can still shift slightly, since server processing time is derived from the same server receive endpoint.
 
 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.