Update forwards option description

Mixing examples and description in the description makes it very noisy
and unfocused.

default.nix

@@ -683,16 +683,18 @@ in
     forwards = mkOption {
       type = with types; attrsOf (either (listOf str) str);
       example = {
-        "user@example.com" = "user@elsewhere.com";
+        "user@example.com" = "user@example.edu";
+        "gamenight@example.com" = [
+          "bob@example.com"
+          "frank@example.org"
+          "wendy@example.net"
+        ];
       };
       description = ''
-        To forward mails to an external address. For instance,
+        Forwards route mail from local addresses to one or more local or external addresses.
-        the value `{ "user@example.com" = "user@elsewhere.com"; }`
+
-        means that mails to `user@example.com` are forwarded to
+        Unlike {option}`mailserver.aliases`, the target addresses cannot send
-        `user@elsewhere.com`. The difference with the
+        mail using the forward address.
-        {option}`mailserver.aliases` option is that `user@elsewhere.com`
-        can't send mail as `user@example.com`. Also, this option
-        allows to forward mails to external addresses.
       '';
       default = { };
     };