Documentation should explain how to use with an explicit AWS Profile

A very common case is using AWS tools using an explicit profile (i.e. a tool has a `--profile ` argument). While using the `AWS_PROFILE` environment variable is often a possibility, there are reasons why this is not always optimal.

The documentation should explain how to use an explicit AWS Profile name. Then presumably the underlying botocore logic will automatically handle all of the different credential sources that can be expressed using the AWS config file and various environment variables, without needing to handle all of those cases individually.