Clarify docs on Timeout::reset

src/reactor/timeout.rs

@@ -49,6 +49,17 @@ impl Timeout {
 
     /// Resets this timeout to an new timeout which will fire at the time
     /// specified by `at`.
+    ///
+    /// This method is usable even of this instance of `Timeout` has "already
+    /// fired". That is, if this future has resovled, calling this method means
+    /// that the future will still re-resolve at the specified instant.
+    ///
+    /// If `at` is in the past then this future will immediately be resolved
+    /// (when `poll` is called).
+    ///
+    /// Note that if any task is currently blocked on this future then that task
+    /// will be dropped. It is required to call `poll` again after this method
+    /// has been called to ensure tha ta task is blocked on this future.
     pub fn reset(&mut self, at: Instant) {
         self.when = at;
         self.token.reset_timeout(self.when, &self.handle);