PBM-1561 Revise known limitations for backups and restores

[radoslawszulgo]

Updated limitations for selective backups and restores, including sharded collections and multi-collection transactions.

[Copilot]

Pull Request Overview

This pull request updates the known limitations documentation for PBM (Percona Backup for MongoDB) selective backups and restores. The changes add clarification about sharded collection restore limitations and reorganize the numbering of existing limitations.

• Added detailed explanation about sharded collection restore limitations when chunk distribution differs
• Renumbered existing limitations to accommodate the new content
• Maintained consistent formatting and structure

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