Merge #3842

3842: Clarify RELAYNETS requirements r=mergify[bot] a=strugee

## What type of PR?

Documentation

## What does this PR do?

Essentially, it puts https://github.com/Mailu/Mailu/issues/369#issuecomment-358071523 into the documentation. For context, I was using the client submission port with a username/password combo _along_ with `RELAYNETS` because I gave up trying to get client auth to work the way that I wanted it, and only fell back to using `RELAYNETS` (with a single IP) as a last-ditch effort to get client mail working.

### Related issue(s)

None

## Prerequisites
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

- [x] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/workflow.html#changelog) entry file.


Co-authored-by: AJ Jordan <alex@strugee.net>

docs/configuration.rst

@@ -84,7 +84,7 @@ account on the server.
 The ``RELAYNETS`` (default: unset) is a comma delimited list of network addresses
 for which mail is relayed for with no authentication required. This should be
 used with great care as misconfigurations may turn your Mailu instance into an
-open-relay!
+open-relay! Make sure clients are using port 25 with no authentication.
 
 The ``RELAYHOST`` is an optional address to use as a smarthost for all outgoing
 mail in following format: ``[HOST]:PORT``. ``RELAYUSER`` and ``RELAYPASSWORD``