Add more detaild comments of maxCacheAge and fix typo
Add more detaild comments of maxCacheAge and fix typo

tualatrix/SDWebImage,dreampiggy/SDWebImage,awkward/SDWebImage,awkward/SDWebImage,dreampiggy/SDWebImage,awkward/SDWebImage,zhang-yawei/SDWebImage,rs/SDWebImage,dreampiggy/SDWebImage