Merge: nixos/nextcloud: document nextcloud-occ command (#422639)

nixos/modules/services/web-apps/nextcloud.md

@@ -56,6 +56,37 @@ it's needed to add them to
 Auto updates for Nextcloud apps can be enabled using
 [`services.nextcloud.autoUpdateApps`](#opt-services.nextcloud.autoUpdateApps.enable).
 
+## `nextcloud-occ` {#module-services-nextcloud-occ}
+
+The management command [`occ`](https://docs.nextcloud.com/server/stable/admin_manual/occ_command.html) can be
+invoked by using the `nextcloud-occ` wrapper that's globally available on a system with Nextcloud enabled.
+
+It requires elevated permissions to become the `nextcloud` user. Given the way the privilege
+escalation is implemented, parameters passed via the environment to Nextcloud (e.g. `OC_PASS`) are
+currently ignored.
+
+Custom service units that need to run `nextcloud-occ` either need elevated privileges
+or the systemd configuration from `nextcloud-setup.service` (recommended):
+
+```nix
+{ config, ... }: {
+  systemd.services.my-custom-service = {
+    script = ''
+      nextcloud-occ …
+    '';
+    serviceConfig = {
+      inherit (config.systemd.services.nextcloud-cron.serviceConfig)
+        User
+        LoadCredential
+        KillMode;
+    };
+  };
+}
+```
+
+Please note that the options required are subject to change. Please make sure to read the
+release notes when upgrading.
+
 ## Common problems {#module-services-nextcloud-pitfalls-during-upgrade}
 
   - **General notes.**