Also call wxAuiManager::UnInit() when manager itself is destroyed

This completes the changes of the previous commit and should ensure that
UnInit() is always called, whoever is destroyed first -- the managed
window or the manager itself.

Also update the documentation to mention that calling UnInit()
explicitly is not necessary any longer.

samples/aui/auidemo.cpp

@@ -116,8 +116,6 @@ public:
             const wxSize& size = wxDefaultSize,
             long style = wxDEFAULT_FRAME_STYLE | wxSUNKEN_BORDER);
 
-    ~MyFrame();
-
     wxAuiDockArt* GetDockArt();
     void DoUpdate();
 
@@ -1016,11 +1014,6 @@ MyFrame::MyFrame(wxWindow* parent,
     m_mgr.Update();
 }
 
-MyFrame::~MyFrame()
-{
-    m_mgr.UnInit();
-}
-
 wxAuiDockArt* MyFrame::GetDockArt()
 {
     return m_mgr.GetArtProvider();