util: Document why ArgsManager (con/de)structor is not inline

src/util/system.cpp

@@ -226,10 +226,11 @@ static bool CheckValid(const std::string& key, const util::SettingsValue& val, u
     return true;
 }
 
-ArgsManager::ArgsManager()
-{
-    // nothing to do
-}
+// Define default constructor and destructor that are not inline, so code instantiating this class doesn't need to
+// #include class definitions for all members.
+// For example, m_settings has an internal dependency on univalue.
+ArgsManager::ArgsManager() {}
+ArgsManager::~ArgsManager() {}
 
 const std::set<std::string> ArgsManager::GetUnsuitableSectionOnlyArgs() const
 {