Add documentation for Timestamping return type

Author: WiSaGaN

src/sys/socket/mod.rs

@@ -563,6 +563,8 @@ pub enum ControlMessageOwned {
     #[cfg(all(target_os = "linux"))]
     ScmTimestampns(TimeSpec),
     /// Configurable nanoseconds resolution timestamps
+    /// Three `TimeSpec`s are returned in the control message. At least one field is non-zero at
+    /// any time. Most timestamps are passed in ts[0]. Hardware timestamps are passed in ts[2].
     ///
     /// [Further reading](https://www.kernel.org/doc/html/latest/networking/timestamping.html)
     #[cfg(all(target_os = "linux"))]