Add kernel version note on event timestamp

Author: mpi3d

src/lib.rs

@@ -922,6 +922,8 @@ impl LineEvent {
     /// The nanosecond timestamp value should are captured
     /// using the `CLOCK_MONOTONIC` offsets in the kernel and
     /// should be compared against `CLOCK_MONOTONIC` values.
+    /// Note that kernel versions prior to 5.7 used
+    /// `CLOCK_REALTIME` offsets instead.
     pub fn timestamp(&self) -> u64 {
         self.0.timestamp
     }