docs: scope server kernel-timestamp claim to this crate's server

irtt-client can talk to any IRTT-compatible peer, not only this
repo's own irtt-server, and has no way to know what clock source
produced a peer-reported wall-clock endpoint. Restrict the
kernel-receive-timestamp-preference claim to when the peer is
this crate's own Linux irtt-server; state plainly that another
peer's reported value is used as-is otherwise.

crates/irtt-client/src/event.rs

@@ -354,13 +354,16 @@ pub struct OneWayDelaySample {
     /// wall time when both are available. The client send endpoint is the
     /// correlated Linux kernel TX timestamp for this send when one is
     /// available and plausible for this send, and the userspace post-send wall
-    /// time otherwise. The server receive endpoint is the server's kernel
-    /// receive timestamp when one is available and plausible, and its
-    /// userspace receive wall time otherwise. `RttSample::raw` is unaffected
-    /// and remains a userspace monotonic measurement, but `adjusted` and
-    /// `effective` can still shift: when the session negotiated wall-clock
-    /// timing rather than monotonic, server processing time is derived from
-    /// this same server receive endpoint.
+    /// time otherwise. The server receive endpoint is whatever wall time the
+    /// peer reported; against this crate's own `irtt-server` on Linux, that is
+    /// its kernel receive timestamp when one is available and plausible, and
+    /// its userspace receive wall time otherwise. Against another
+    /// IRTT-compatible peer, the client has no way to know which clock source
+    /// produced it. `RttSample::raw` is unaffected and remains a userspace
+    /// monotonic measurement, but `adjusted` and `effective` can still shift:
+    /// when the session negotiated wall-clock timing rather than monotonic,
+    /// server processing time is derived from this same server receive
+    /// endpoint.
     pub client_to_server: Option<SignedDuration>,
     /// Server-to-client delay.
     ///