TASK-91084: adjust override documentation

[rafaelmatsumotomb]

---

Note

Updates Transmissions API docs to clarify override behavior and add a new header option.

• Clarifies override.from.name fallback to content.from.name or stored template from.name
• Documents new override.list_id to set the List-Id header (RFC 2919) and adds it to the request sample

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[brandonglesmann]

LGTM