Remove $server_port from the Host header in the nginx reverse proxy example (#20391)

Fixes #19583

The nginx example in `docs/reverse_proxy.md` sets `proxy_set_header Host
$host:$server_port;`. The server block listens on 443, so nginx sends
`Host: matrix.example.com:443`. With `public_baseurl:
https://matrix.example.com/`, the request URI never starts with
`public_baseurl`, and `SsoRedirectServlet` keeps redirecting to itself.
That breaks SSO even when the internal and external ports are the same.
The line came in with #17994.

This switches the example to `proxy_set_header Host $host;`, as
@anoadragon453 recommended in #18359. I added a short comment for setups
where clients use a non-default port: use `$http_host` and put that port
in `public_baseurl`. The setup in #18359 (external port 50080) needs
that variant.

I left `docker/conf-workers/nginx.conf.j2` alone. Its listener port
matches `public_baseurl`, so `$host:$server_port` is right there.

This is a docs-only change, so there are no tests.

Thanks to @shibotto for reporting this and for pointing at the fix.

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* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
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The entry should:
- Be a short description of your change which makes sense to users.
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* [x] [Code
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correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

changelog.d/20391.doc

@@ -0,0 +1 @@
+Remove `$server_port` from the `Host` header in the nginx reverse proxy example, as it can cause an SSO redirect loop.

docs/reverse_proxy.md

@@ -76,7 +76,9 @@ server {
         proxy_pass http://localhost:8008;
         proxy_set_header X-Forwarded-For $remote_addr;
         proxy_set_header X-Forwarded-Proto $scheme;
-        proxy_set_header Host $host:$server_port;
+        # If clients reach Synapse on a non-default port, use `$http_host` instead
+        # so the port is kept, and include that port in `public_baseurl`.
+        proxy_set_header Host $host;
 
         # Nginx by default only allows file uploads up to 1M in size
         # Increase client_max_body_size to match max_upload_size defined in homeserver.yaml