docs: link to improved Uploads Migrate Rake Task

What does this MR do?

Improve details on moving uploads to object storage via Uploads Migrate Rake Task. Make better known changes added in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21779.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards