Documentation - backup multipart upload

We need to document the backup multipart upload -> gitlab_rails['backup_multipart_chunk_size'] in gitlab.rb

Follow-up from: omnibus-gitlab!375 (merged)

Implementation Guide

Document under https://docs.gitlab.com/ee/raketasks/backup_gitlab.html#using-amazon-s3 the setting gitlab_rails['backup_multipart_chunk_size'] = 123456.