pageserver: update upload queue TODOs (#11377)

Update some upload queue TODOs, particularly to track
https://github.com/neondatabase/neon/issues/10283, which I won't get
around to.

pageserver/src/tenant/remote_timeline_client.rs

@@ -1968,9 +1968,7 @@ impl RemoteTimelineClient {
     /// Pick next tasks from the queue, and start as many of them as possible without violating
     /// the ordering constraints.
     ///
-    /// TODO: consider limiting the number of in-progress tasks, beyond what remote_storage does.
-    /// This can launch an unbounded number of queued tasks. `UploadQueue::next_ready()` also has
-    /// worst-case quadratic cost in the number of tasks, and may struggle beyond 10,000 tasks.
+    /// The number of inprogress tasks is limited by `Self::inprogress_tasks`, see `next_ready`.
     fn launch_queued_tasks(self: &Arc<Self>, upload_queue: &mut UploadQueueInitialized) {
         while let Some((mut next_op, coalesced_ops)) = upload_queue.next_ready() {
             debug!("starting op: {next_op}");
@@ -2218,6 +2216,11 @@ impl RemoteTimelineClient {
                     }
                     res
                 }
+                // TODO: this should wait for the deletion to be executed by the deletion queue.
+                // Otherwise, the deletion may race with an upload and wrongfully delete a newer
+                // file. Some of the above logic attempts to work around this, it should be replaced
+                // by the upload queue ordering guarantees (see `can_bypass`). See:
+                // <https://github.com/neondatabase/neon/issues/10283>.
                 UploadOp::Delete(delete) => {
                     if self.config.read().unwrap().block_deletions {
                         let mut queue_locked = self.upload_queue.lock().unwrap();