Update reverse proxy docs with what we've learned from #17986 (#17994)

Update reverse proxy docs with what we've learned from
https://github.com/element-hq/synapse/pull/17986

Also vice versa and update our nginx config with what I learned from the
reverse proxy docs.

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
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.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

1 files changed, 1 insertions, 1 deletions

diff --git a/docs/reverse_proxy.md b/docs/reverse_proxy.md

index 7128af114e..45de2b1f65 100644
--- a/docs/reverse_proxy.md
+++ b/docs/reverse_proxy.md
@@ -74,7 +74,7 @@ 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;
+        proxy_set_header Host $host:$server_port;
 
         # 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