add allow_username_mismatch and explanation

src/content/docs/ispmail-trixie/310-prevent-spoofing-dkim.mdx

@@ -288,6 +288,7 @@ configuration file:
 cat > /etc/rspamd/local.d/dkim_signing.conf << 'EOF'
 path = "/var/lib/rspamd/dkim/$domain.$selector.key";
 selector_map = "/etc/rspamd/dkim_selectors.map";
+allow_username_mismatch = true;
 EOF
 
 # Restart rspamd
@@ -305,6 +306,16 @@ echo example.org 2025100901 > /etc/rspamd/dkim_selectors.map
 That’s all. rspamd now knows that whenever it sees an outgoing email from `anyone@example.org` it will get the DKIM
 private key from /var/lib/rspamd/dkim/**example.org**.**2025100901**.key and use it to sign the email.
 
+The `allow_username_mismatch` setting makes rspamd sign an email even if the
+authenticated user (during relaying) does not match the actual sender address.
+Sounds complicated? Well, the use case is _aliases_. Imagine that
+`john@example.com` has an alias address of `pizza@example.org`. If John logs in
+as `john@example.org` but chooses to set his sender address to
+`pizza@example.org` then rspamd will (by default) not add a DKIM signature due
+to a mismatch. Setting `allow_username_mismatch` overrides this behavior and
+John's email will be signed even if he sends on behalf of `pizza@example.org`.
+(Thanks, DBL, for the hint.)
+
 ### Send a test email
 
 You could either just send an email from your mail client (or Roundcube) through your mail server to another email