document WaitableCondvar (#24252)

runtime/src/waitable_condvar.rs

@@ -12,12 +12,16 @@ pub struct WaitableCondvar {
 }
 
 impl WaitableCondvar {
+    /// wake up all threads waiting on this event
     pub fn notify_all(&self) {
         self.event.notify_all();
     }
+    /// wake up one thread waiting on this event
     pub fn notify_one(&self) {
         self.event.notify_one();
     }
+    /// wait on the event
+    /// return true if timed out, false if event triggered
     pub fn wait_timeout(&self, timeout: Duration) -> bool {
         let lock = self.mutex.lock().unwrap();
         let res = self.event.wait_timeout(lock, timeout).unwrap();