Move content_panels inside BookPage definition.

docs/topics/snippets.rst

@@ -110,11 +110,10 @@ In the above example, the list of adverts is a fixed list, displayed as part of
           related_name='+'
       )
   
-  
-  BookPage.content_panels = [
-      SnippetChooserPanel('advert'),
-      # ...
-  ]
+      content_panels = Page.content_panels + [
+          SnippetChooserPanel('advert'),
+          # ...
+      ]
 
 
 The snippet could then be accessed within your template as ``page.advert``.
@@ -152,11 +151,10 @@ To attach multiple adverts to a page, the ``SnippetChooserPanel`` can be placed
   class BookPage(Page):
       ...
   
-  
-  BookPage.content_panels = [
-      InlinePanel('advert_placements', label="Adverts"),
-      # ...
-  ]
+      content_panels = Page.content_panels + [
+          InlinePanel('advert_placements', label="Adverts"),
+          # ...
+      ]