[Atlas] Statement that Atlas snapshots with Encryption At Rest enabled cannot be downloaded is no longer true

Description

https://docs.atlas.mongodb.com/backup/cloud-backup/restore/#restore-a-snapshot-of-a-cluster-with-encryption-at-rest

Highlighted text is no longer true - it is possible to download encrypted snapshots in Atlas (and use the KMIP proxy tool to decrypt them).

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)