Mention how to redirect the Jaeger traces to a specific Jaeger instance

[MTRNord]

This adds a small note to the opentracing docs when your jaeger is not on localhost next to synapse. For example in a Kubernetes env.

Signed-Off-By: Marcel Radzio support@nordgedanken.dev

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.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

[DMRobertson]

See https://github.com/matrix-org/synapse/pull/16531/files#r1368501309. Would like to see moving the sentence at least, everything else is nice-to-have.

[DMRobertson]

Many thanks!