If any party to a suit gives evidence on his own behalf, or requires the attendance and examination of any such advocate or person referred to in this section, he shall be deemed to have consented to such disclosure and any evidence given in consequence by such party or his agent shall be treated as a waiver of the privilege; but the mere fact that a party has submitted to be cross-examined, or that he has submitted a document for identification, shall not be treated as a waiver of the privilege.