Document IAM Profile AWS S3 configuration key.

What does this MR do?

Document additional parameters available to the backup rake job configuration.

Are there points in the code the reviewer needs to double check?

Probably not.

Why was this MR needed?

Un-documented configuration options

What are the relevant issue numbers?

#12756 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together