doc: adds max manifest consideration to manifest documentation

[imdominicreed]

Proposed Changes

• document max manifest size for large TDFs

Checklist

• A clear description of the change has been included in this PR.
• A clear description of whether this change is a Major, Minor, Patch or cosmetic change as per the Versioning Guidelines has been included in this PR.
• All schema validation tests have been updated appropriately and are passing.
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• MAJOR/MINOR VERSION CHANGES ONLY: A writeup has been included discussing the motivation and impact of this change.
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• DRAFT MERGE ONLY: Version numbers have been updated as per the Versioning Guidelines.
• This change otherwise adheres to the project Contribution Guidelines.