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

doc/raketasks/backup_restore.md

@@ -79,6 +79,9 @@ gitlab_rails['backup_upload_connection'] = {
   'region' => 'eu-west-1',
   'aws_access_key_id' => 'AKIAKIAKI',
   'aws_secret_access_key' => 'secret123'
+  # If using an IAM Profile, leave aws_access_key_id & aws_secret_access_key empty
+  # ie. 'aws_access_key_id' => '',
+  # 'use_iam_profile' => 'true'
 }
 gitlab_rails['backup_upload_remote_directory'] = 'my.s3.bucket'
 ```
@@ -95,6 +98,9 @@ For installations from source:
         region: eu-west-1
         aws_access_key_id: AKIAKIAKI
         aws_secret_access_key: 'secret123'
+        # If using an IAM Profile, leave aws_access_key_id & aws_secret_access_key empty
+        # ie. aws_access_key_id: ''
+        # use_iam_profile: 'true'
       # The remote 'directory' to store your backups. For S3, this would be the bucket name.
       remote_directory: 'my.s3.bucket'
       # Turns on AWS Server-Side Encryption with Amazon S3-Managed Keys for backups, this is optional