TASK-91084: add override field to transmissions docs

[rafaelmatsumotomb]

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Note

Adds override support docs to transmissions for overriding from and reply_to, with a warning that override does not support substitution_data, and updates the sample payload.

• Transmissions API docs (content/api/transmissions.apib):
  • New override block in request body:
    • from (string or object with name, email) and reply_to.
  • Warning: override does not support substitution_data.
  • Sample payload: includes override with from and reply_to.

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