Update documented Geo limitations

What does this MR do?

Per #328295 (comment 643285932), we incorrectly state in the documentation that we do not de-duplicate repositories on Geo secondary sites. This MR updates the limitations to clarify that deduplication is run on a secondary.

Related issues

#328295 (closed)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Ensure that the product tier badge is added to doc's h1.
• Request a review based on the documentation page's metadata and associated Technical Writer.

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Review checklist

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