nixos/nextcloud: improve documentation of adminuser-option

This option only has an effect on the initial setup of Nextcloud and
changes later won't have any effect.

Same issue as with `adminpassFile` - it's only passed to the installer
command - but even worse because the username is frequently used as
unique ID in the database, so there's no trivial way to change it, even
imperatively.

(cherry picked from commit 648546b09d)

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

@@ -388,7 +388,11 @@ in {
       adminuser = mkOption {
         type = types.str;
         default = "root";
-        description = lib.mdDoc "Username for the admin account.";
+        description = lib.mdDoc ''
+          Username for the admin account. The username is only set during the
+          initial setup of Nextcloud! Since the username also acts as unique
+          ID internally, it cannot be changed later!
+        '';
       };
       adminpassFile = mkOption {
         type = types.str;