nixos/release-notes: add entry for services.prosody.checkConfig

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -47,6 +47,8 @@ The pre-existing [services.ankisyncd](#opt-services.ankisyncd.enable) has been m
   existing process, but will need to start that process from gdb (so it is a
   child). Or you can set `boot.kernel.sysctl."kernel.yama.ptrace_scope"` to 0.
 
+- `prosody` gained a config check option named `services.prosody.checkConfig` which runs `prosodyctl check config` and is turned on by default.
+
 - Gitea 1.21 upgrade has several breaking changes, including:
   - Custom themes and other assets that were previously stored in `custom/public/*` now belong in `custom/public/assets/*`
   - New instances of Gitea using MySQL now ignore the `[database].CHARSET` config option and always use the `utf8mb4` charset, existing instances should migrate via the `gitea doctor convert` CLI command.