docs(configuration): add APIS_FORMER_BASE_URIS setting

docs/source/configuration.rst

@@ -14,6 +14,8 @@ comes from the ``APIS_BASE_URI`` setting. The second part comes from the reverse
 route of ``GetEntityGenericRoot`` if that exists. If not, it uses the
 the reverse route of ``apis_core:GetEntityGeneric``, which is defined in
 :py:mod:`apis_core.urls` and defaults to ``/entity/{pk}``.
+If the value of ``APIS_BASE_URL`` changes and urls containing the former ``APIS_BASE_URL`` remain in the database, then the old ``APIS_BASE_URL`` value must added to the setting ``APIS_FORMER_BASE_URIS``.
+``APIS_FORMER_BASE_URIS`` is a list that contains base urls that were once used in this APIS instance.
 
 Django Settings
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