Provide instructions on S3 bucket name in StorageManager config

StorageManager config gives some instructions how to fill the necessary values.

In the S3 section there is a field for the bucket name. For AWS at least, StorageManager expects this to be the bucket name only, without the domain part; however, in certain cases (like the AWS API) retrieving the bucket name yields the full name, including the domain. Using this full name will make CS fail with cryptic messages about invalid certificates (which is correct per se, but does not help troubleshoot the problem).

I believe it will be helpful to amend the text with a note on this, saying that (for AWS at least) the bucket name should be the name itself, excluding any domain part.