Replaced outdated reference to MPTT with correct django-treebeard link (

#5818)

docs/conf.py

@@ -36,6 +36,7 @@
     'django': ('https://docs.djangoproject.com/en/dev/', 'http://docs.djangoproject.com/en/dev/_objects/'),
     'classytags': ('https://django-classy-tags.readthedocs.io/en/latest/', None),
     'sekizai': ('https://django-sekizai.readthedocs.io/en/latest/', None),
+    'treebeard': ('https://django-treebeard.readthedocs.io/en/latest/', None),
 }
 
 # Add any paths that contain templates here, relative to this directory.

docs/reference/api.rst

@@ -104,7 +104,8 @@ input-checking.
     :param plugin_type: What type of plugin to add
     :type plugin_type: string or :class:`cms.plugin_base.CMSPluginBase` sub-class, must be a valid plugin
     :param string language: Language code for this plugin, must be in :setting:`django:LANGUAGES`
-    :param string position: Position to add this plugin to the placeholder
+    :param string position: Position to add this plugin to the placeholder, must be a valid django-treebeard ``pos``
+        value for :meth:`treebeard:treebeard.models.Node.add_sibling`
     :param target: Parent plugin. Must be plugin instance
     :param data: Data for the plugin type instance