Update installation.md to fix a confusion that whether synapse can serve Client Well-Known URI

[matrixbot]

This issue has been migrated from #16678.

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Actually, by my test, Synapse can serve Client Well-Known URI once you set public_baseurl in homeserver.yaml. But current document seems unclear on this and make newbies feel "Synapse can't serve Client Well-Known URI and I must use a reverse proxy to do it".

In fact, I was stuck in it at first...

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)

Signed-off-by: LiAlH4 LiAlH4_Tr@Outlook.com

[djmaze]

Whats more, synapse should probably also be able to output the org.matrix.msc3575.proxy at the well-known url. Otherwise we still need a reverse proxy configuration for Element X.