Improve fd_num comment

Co-authored-by: Ralf Jung <post@ralfj.de>

src/shims/unix/linux/epoll.rs

@@ -51,8 +51,9 @@ impl EpollEventInstance {
 #[derive(Clone, Debug)]
 pub struct EpollEventInterest {
     /// The file descriptor value of the file description registered.
-    /// This is only used as the key of ready_list, so the exact value is stored
-    /// instead of reference.
+    /// This is only used for ready_list, to inform userspace which FD triggered an event.
+    /// For that, it is crucial to preserve the original FD number.
+    /// This FD number must never be "dereferenced" to a file description inside Miri.
     fd_num: i32,
     /// The events bitmask retrieved from `epoll_event`.
     events: u32,