docs(storage): document MVCC history retention

[tsdebug]

Summary

• Document the current MVCC history-retention behavior.
• Explain that document and index revisions are preserved as append-only history.
• Clarify that transactor pruning only removes in-memory commit metadata.
• Distinguish client outbox retention from document revision history.
• Explain storage planning for write-heavy applications.

This is a documentation-only change. Retention controls and background history cleanup are not yet implemented and will be addressed in a separate change.

Closes #45

Validation

• bun run check:docs-wrap
• git diff --check

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