chore: Raise awareness of v13 breaking change better

Several issues have been raised where this was not an obvious breaking change to the reader. Additional context on impact relevance has been included.

CHANGELOG.md

@@ -153,6 +153,7 @@ This patch release fixes two bugs that Rspamd users encountered with the `v13.0.
 - **Postfix:**
   - `/etc/postfix/master.cf` has renamed the "smtps" service to "submissions" ([#3235](https://github.com/docker-mailserver/docker-mailserver/pull/3235))
     - This is the modern `/etc/services` name for port 465, aligning with the similar "submission" port 587.
+    - If you have configured Proxy Protocol support with a reverse proxy via `postfix-master.cf` (_as [per our docs guide](https://docker-mailserver.github.io/docker-mailserver/v13.0/examples/tutorials/mailserver-behind-proxy/)_), you will want to update `smtps` to `submissions` there.
   - Postfix now defaults to supporting DSNs (_[Delivery Status Notifications](https://github.com/docker-mailserver/docker-mailserver/pull/3572#issuecomment-1751880574)_) only for authenticated users (_via ports 465 + 587_). This is a security measure to reduce spammer abuse of your DMS instance as a backscatter source. ([#3572](https://github.com/docker-mailserver/docker-mailserver/pull/3572))
     - If you need to modify this change, please let us know by opening an issue / discussion.
     - You can [opt out (_enable DSNs_) via the `postfix-main.cf` override support](https://docker-mailserver.github.io/docker-mailserver/v12.1/config/advanced/override-defaults/postfix/) using the contents: `smtpd_discard_ehlo_keywords =`.