Fix and extend persistent objects overview in the manual

Give the example of saving the frame geometry, which is undoubtedly the
most common use case of these classes, and simplify the existing
example with wxNotebook.

docs/doxygen/overviews/persistence.h

@@ -56,9 +56,10 @@ Currently the following classes are supported:
 To automatically save and restore the properties of the windows of classes
 listed above you need to:
 
--# Set a unique name for the window using wxWindow::SetName(): this step is
-   important as the name is used in the configuration file and so must be
-   unique among all windows of the same class.
+-# Set a unique name for the window by either using this name in its
+   constructor or calling wxWindow::SetName(): this step is important as the
+   name is used in the configuration file and so must be unique among all
+   windows of the same class.
 -# Call wxPersistenceManager::Register() at any moment after creating the
    window and then wxPersistenceManager::Restore() when the settings may be
    restored (which can't be always done immediately, e.g. often the window
@@ -72,8 +73,37 @@ listed above you need to:
    Otherwise the settings will be automatically saved when the control itself
    is destroyed.
 
-Example of using a notebook control which automatically remembers the last open
-page:
+A convenient wxPersistentRegisterAndRestore() helper function can be used to
+perform the first steps at once, e.g. to automatically save and restore the
+position of a @c MyFrame window in your application, you need to only do the
+following:
+
+@code
+#include <wx/persist/toplevel.h>
+
+...
+
+MyFrame::MyFrame(wxWindow* parent, ...)
+       : wxFrame(parent, ...)
+{
+    ... all the other initialization ...
+
+    // Restore the previously saved geometry, if any, and register this frame
+    // for its geometry to be saved when it is closed using the given wxConfig
+    // key name.
+    if ( !wxPersistentRegisterAndRestore(this, "my_frame_name") )
+    {
+        // Choose some custom default size for the first run -- or don't do
+        // anything at all and let the system use the default initial size.
+        SetClientSize(FromDIP(wxSize(800, 600)));
+    }
+
+    Show(true);
+}
+@endcode
+
+And here is an example of using a notebook control which automatically
+remembers the last open page without using the helper function:
 
 @code
     wxNotebook *book = new wxNotebook(parent, wxID_ANY);
@@ -81,11 +111,10 @@ page:
     book->AddPage(...);
     book->AddPage(...);
     book->AddPage(...);
-    if ( !wxPersistenceManager::RegisterAndRestore(book) )
-    {
-        // nothing was restored, so choose the default page ourselves
-        book->SetSelection(0);
-    }
+
+    // We don't check for the return value here as the first page is selected
+    // by default anyhow and we just keep this selection in this case.
+    wxPersistenceManager::Get().RegisterAndRestore(book);
 @endcode