A party may serve upon any other party a written request for the admission, for purposes of the pending action only, of the truth of any matters within the scope of Articles 1422 through 1425 set forth in the request or of the truth of any relevant matters of fact, including the genuineness of any documents described in the request. Copies of documents shall be served with the request unless they have been or are otherwise furnished or made available for inspection and copying. The request may, without leave of court, be served upon the plaintiff after commencement of the action and upon any other party with or after service of the petition upon that party.