docs(admin): Document MultipartUpload for uploading chunks to s3

[joshtrichards]

☑️ Resolves

Adds a clarifying note to the "Large file upload on object storage" section about MultipartUpload support that was added awhile back in nextcloud/server#27034.

Aside: This entire chapter could use some love to make it more cohesive and approachable - due to all the additions made over time - but I don't have time to edit the entire chapter right now. :) So, for now, just wanted to at least get this in there so it's covered.

🖼️ Screenshots

[github-actions]

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[susnux]

/backport to stable29

[susnux]

/backport to stable28