Airbnb Inc. issued a better-than-expected outlook for the third quarter as it saw encouraging summer travel demand, but warned that growth rates may not keep up later this year due to tough year-ago comparisons.For the three months ending Sept. 30, the short-term rental company said growth on the basis of nights and seats booked — a closely watched metric — is expected to be “relatively stable” from the 7.4% achieved in the second quarter, Airbnb said in a shareholder letter on Wednesday. That’s...