[wgpu-core] document which closures are guaranteed to be called

wgpu-core/src/device/global.rs

@@ -2083,7 +2083,7 @@ impl Global {
         self.hub.devices.remove(device_id);
     }
 
-    /// This closure will be called exactly once during "lose the device".
+    /// `device_lost_closure` might never be called.
     pub fn device_set_device_lost_closure(
         &self,
         device_id: DeviceId,
@@ -2144,6 +2144,7 @@ impl Global {
         self.hub.queues.remove(queue_id);
     }
 
+    /// `op.callback` is guaranteed to be called.
     pub fn buffer_map_async(
         &self,
         buffer_id: id::BufferId,

wgpu-core/src/device/queue.rs

@@ -1396,6 +1396,7 @@ impl Queue {
         unsafe { self.raw().get_timestamp_period() }
     }
 
+    /// `closure` is guaranteed to be called.
     pub fn on_submitted_work_done(
         &self,
         closure: SubmittedWorkDoneClosure,