Document aws sessionToken for temporary credentials

[bentsherman]

This commit -- https://github.com/nextflow-io/nextflow/commit/15ac3938542f8995f88f04b9b32ff59ec161c040 -- added support for AWS temporary credentials via the aws.sessionToken option. Needs to be documented.

[pditommaso]

I think sessionToken never worked. Better it works, but it's not refreshed automatically, therefore it's quite useless.

[bentsherman]

@gwright99 discovered it while trying to help a client with temp credentials. He's still trying to get it to work but I'm not sure if the client in question expects Nextflow to refresh automatically or if they will do it themselves.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.