You wouldn’t know it by looking at this year’s returns, but the performances of the average emerging-market and foreign-stock mutual funds have run well ahead of the S&P 500’s in the past 15 years. Investors in the average foreign-stock fund would have turned a $10,000 investment made on Sept. 30, 1999, into $25,277 by Nov. 18 of this year, according to Morningstar Inc. data.