docs: document client public API

crates/irtt-client/src/timing.rs

@@ -1,12 +1,20 @@
 use std::time::{Instant, SystemTime};
 
+/// Client-side timestamp captured in both wall-clock and monotonic time.
+///
+/// Wall-clock time is used for protocol timestamp comparison and one-way delay
+/// calculations. Monotonic time is used for elapsed client-side timing such as
+/// RTT, scheduling, and timeouts.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct ClientTimestamp {
+    /// System wall-clock time.
     pub wall: SystemTime,
+    /// Monotonic timestamp sampled alongside [`wall`](Self::wall).
     pub mono: Instant,
 }
 
 impl ClientTimestamp {
+    /// Capture the current wall-clock and monotonic timestamps.
     pub fn now() -> Self {
         Self {
             wall: SystemTime::now(),