Update aws_s3.rst with clarifications re service setup
Update aws_s3.rst with clarifications re service setup

After struggling to set up LiipImagineBundle in an AWS EC2 environment where we do not want to use hard coded AWS API credentials, but instead rely on attaching an IAM role to our EC2 instances, I realized that the current documentation lacks some relevant information.
liip/LiipImagineBundle,liip/LiipImagineBundle