Improve documentation for /_synapse/admin/v1/rooms/<room_id>/timestamp_to_event

[Half-Shot]

It returns an extra timestamp parameter, and also the event_id description was a little confused.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
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• Pull request includes a sign off
• Code style is correct
  (run the linters)

[clokep]

Seems to match the spec https://spec.matrix.org/v1.8/client-server-api/!