API-33 docs: add POST /v1/reversal endpoint documentation

[ecunado]

Summary

• Adds ## Reversal Operations section to restendpoints.md documenting the new POST /v1/reversal endpoint
• Adds ## Reversal section to restobjects.md with the ViscusReversalRequest object, full messageReasonCode enum table (27 values), and code examples
• Follows the same documentation pattern as the existing Batch and MOTO operations sections

Changes

• restapi/restendpoints.md — new Reversal Operations section with parameters, return codes, behaviour examples (full, partial, with reason code, validation error), and a code example
• restapi/restobjects.md — new Reversal section with ViscusReversalRequest properties, complete messageReasonCode enum table, and three JSON code examples

Test plan

• Review rendered Docusaurus output for correct formatting
• Verify all internal links (restobjects#viscusReversalRequest, restendpoints#reversal-operations) resolve correctly
• Confirm enum table renders properly in the docs site

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