Added comments explaining signatureCache config option.
Added comments explaining signatureCache config option.

aws/aws-sdk-js,chrisradek/aws-sdk-js,AdityaManohar/aws-sdk-js,GlideMe/aws-sdk-js,AdityaManohar/aws-sdk-js,grimurjonsson/aws-sdk-js,aws/aws-sdk-js,mapbox/aws-sdk-js,grimurjonsson/aws-sdk-js,chrisradek/aws-sdk-js,LiuJoyceC/aws-sdk-js,AdityaManohar/aws-sdk-js,aws/aws-sdk-js,mapbox/aws-sdk-js,guymguym/aws-sdk-js,mapbox/aws-sdk-js,LiuJoyceC/aws-sdk-js,aws/aws-sdk-js,jeskew/aws-sdk-js,GlideMe/aws-sdk-js,GlideMe/aws-sdk-js,grimurjonsson/aws-sdk-js,guymguym/aws-sdk-js,guymguym/aws-sdk-js,jeskew/aws-sdk-js,chrisradek/aws-sdk-js,jeskew/aws-sdk-js,LiuJoyceC/aws-sdk-js,jeskew/aws-sdk-js,guymguym/aws-sdk-js,GlideMe/aws-sdk-js,chrisradek/aws-sdk-js