add comment

src/server/service/acl.js

@@ -29,6 +29,8 @@ class AclService {
 
     const guestMode = this.configManager.getConfig('crowi', 'security:restrictGuestMode');
 
+    // 'Readonly' => returns true (allow access to guests)
+    // 'Deny', null, undefined, '', ... everything else => returns false (requires login)
     return guestMode === this.labels.SECURITY_RESTRICT_GUEST_MODE_READONLY;
   }