TAIPEI, July 21 (Reuters) - Taiwan is investigating a potential leak of official documents including diplomatic cables and classified reports on the island's sensitive bid to join a global trade pact, according to two officials familiar with the probe. One official said initial findings show parts of the documents, posted on online message board 8kun and reviewed by Reuters, are real while bits were forged, without giving details. A second official said parts of the documents appeared to be "authentic" and they could not immediately determine the origin of the documents shared on the internet. The officials asked not to...