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))

changelog.d/17994.doc

@@ -0,0 +1 @@
+Fix example in reverse proxy docs to include server port.

docker/conf-workers/nginx.conf.j2

@@ -38,6 +38,9 @@ server {
 {% if using_unix_sockets %}
         proxy_pass http://unix:/run/main_public.sock;
 {% else %}
+        # note: do not add a path (even a single /) after the port in `proxy_pass`,
+        # otherwise nginx will canonicalise the URI and cause signature verification
+        # errors.
         proxy_pass http://localhost:8080;
 {% endif %}
         proxy_set_header X-Forwarded-For $remote_addr;

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