Docs: Fix some Doxygen warnings

modules/juce_audio_plugin_client/utility/juce_LinuxMessageThread.h

@@ -33,6 +33,7 @@ namespace juce
 // Implemented in juce_linux_Messaging.cpp
 bool dispatchNextMessageOnSystemQueue (bool returnIfNoPendingMessages);
 
+/** @internal */
 class MessageThread
 {
 public:

modules/juce_gui_extra/misc/juce_WebBrowserComponent.h

@@ -144,6 +144,7 @@ public:
 protected:
     friend class WindowsWebView2WebBrowserComponent;
 
+    /** @internal */
     struct ConstructWithoutPimpl
     {
         explicit ConstructWithoutPimpl (bool unloadOnHide) : unloadWhenHidden (unloadOnHide) {}