message: Clarify semantics of setting an error on AsyncBufferTransferManager.
diff: @@ -287,6 +287,9 @@ class PjRtClient {
  // complete. Puts all buffers in an error state. For the stream executor
  // client, since error states are not well supported, this triggers a fatal
  // error.
  + //
  + // SetTransferError may be called at most once, and may not be called unless
  + // at least one buffer has not yet had its final transfer initiated.
  virtual void SetTransferError(Status error) = 0;
  };
  