(Reuters) -Saudi Arabiahas detained 21 Iranian nationals who were aboard two boats near an island off the kingdom's eastern coast, the Saudi border guard said on Thursday. Separated by about 250 km (150 miles) of Gulf waters, Shi'ite Muslim powerIranand Sunni-ledSaudi Arabiahave often had tense relations. U.S. ally Saudi Arabia has accusedIranof fomenting ...