nixos/mastodon: clarify the need to set streamingProcesses

Explicitly declaring that option is now necessary, but wasn't in the module shipped with 23.05.

nixos/doc/manual/release-notes/rl-2311.section.md

@@ -71,7 +71,9 @@ Make sure to also check the many updates in the [Nixpkgs library](#sec-release-2
 - `services.mastodon` doesn't support providing a TCP port to its `streaming`
   component anymore, as upstream implemented parallelization by running
   multiple instances instead of running multiple processes in one instance.
-  Please create a PR if you are interested in this feature.
+  Please create a PR if you are interested in this feature.\
+  Due to this, the desired number of such instances
+  {option}`services.mastodon.streamingProcesses` now needs to be declared explicitly.
 
 - The `services.hostapd` module was rewritten to support `passwordFile` like
   options, WPA3-SAE, and management of multiple interfaces. This breaks

nixos/modules/services/web-apps/mastodon.nix

@@ -229,7 +229,7 @@ in {
       streamingProcesses = lib.mkOption {
         description = lib.mdDoc ''
           Number of processes used by the mastodon-streaming service.
-          Recommended is the amount of your CPU cores minus one.
+          Please define this explicitly, recommended is the amount of your CPU cores minus one.
         '';
         type = lib.types.ints.positive;
         example = 3;