add recommended comment

Author: zhouselena

x/mongo/driver/topology/pool.go

@@ -503,6 +503,13 @@ func (p *pool) checkOut(ctx context.Context) (conn *connection, err error) {
 		}
 		return nil, ErrPoolClosed
 	case poolPaused:
+		// Wrap poolCleared in a driver.Error so we can add the
+		// "TransientTransactionError" label. This will add
+		// "TransientTransactionError" to all poolClearedError instances, not
+		// just those that happened during transactions. While that behavior is
+		// different than other places we add "TransientTransactionError", it is
+		// consistent with the Transactions specification and simplifies the
+		// code.
 		pcErr := poolClearedError{err: p.lastClearErr, address: p.address}
 		err := driver.Error{
 			Message: pcErr.Error(),