Docs review: MR diffs external storage

What does this MR do?

Post-merge docs review: MR diffs external storage.

Related issues

• Original MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24276
• Feature introduced by https://gitlab.com/gitlab-org/gitlab-ce/issues/52568
• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57335

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

• Dev's review