Merge branch 'patch-3' into 'master'

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

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [X] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5910

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