Apply suggestions from code review

Co-authored-by: Adrian Garcia Badaracco <1755071+adriangb@users.noreply.github.com>

Author: alamb

datafusion/datasource/src/file_stream/scan_state.rs

@@ -111,8 +111,14 @@ impl ScanState {
                 Poll::Pending => {
                     self.pending_open = Some(PendingOpen { planner, io_future });
                     return ScanAndReturn::Return(Poll::Pending);
-                }
+                    // We polled the IO future but it didn't complete
+                    // Back to the same state and wait until the next round of polling
+                    self.pending_open = Some(PendingOpen { planner, io_future });
+                    return ScanAndReturn::Return(Poll::Pending);
                 Poll::Ready(Ok(())) => {
+                    // We polled the file open IO future and it completed.
+                    // It yielded us a `MorselPlanner` which we store.
+                    // Now we can move onto polling the next file open.
                     self.ready_planners.push_back(planner);
                     return ScanAndReturn::Continue;
                 }