Improve wxIMPLEMENT_APP() documentation

Explain that this macro defines the application entry point.

Also document wxIMPLEMENT_WXWIN_MAIN which wasn't documented at all
previously.

docs/doxygen/overviews/app.h

@@ -53,12 +53,9 @@ bool DerivedApp::OnInit()
 }
 @endcode
 
-Note the use of wxIMPLEMENT_APP(appClass), which allows wxWidgets to dynamically
-create an instance of the application object at the appropriate point in
-wxWidgets initialization. Previous versions of wxWidgets used to rely on the
-creation of a global application object, but this is no longer recommended,
-because required global initialization may not have been performed at
-application object construction time.
+Note the use of wxIMPLEMENT_APP(), which defines the application entry
+point (either @c main() or @c WinMain() function, depending on the platform)
+and tells wxWidgets which application class should be used.
 
 You can also use wxDECLARE_APP(appClass) in a header file to declare the wxGetApp
 function which returns a reference to the application object. Otherwise you can