Add upgrade consideration note for change to BaseSetting

docs/reference/contrib/settings.md

@@ -1,8 +1,7 @@
 # Settings
 
 The `wagtail.contrib.settings` module allows you to define models that hold
-settings which are either generic (that is the same) across all `Site`s or
-specific to each `Site`.
+settings which are either common across all site records, or specific to each site.
 
 Settings are editable by administrators within the Wagtail admin, and can be
 accessed in code as well as in templates.

docs/releases/4.0.md

@@ -364,3 +364,25 @@ As part of the page editor redesign, we have removed support for the `classname=
 -   This is an optional replacement, there are no immediate plans to remove the `route` decorator at this time.
 -   The `RoutablePageMixin` contrib module now provides a `path` decorator that behaves the same way as Django's [`django.urls.path`](https://docs.djangoproject.com/en/stable/ref/urls/#django.urls.path) function.
 -   `RoutablePageMixin`'s `route` decorator will now redirect to a new `re_path` decorator that emulates the behaviour of [`django.urls.re_path`](https://docs.djangoproject.com/en/stable/ref/urls/#django.urls.re_path).
+
+### `BaseSetting` model replaced by `BaseSiteSetting`
+
+The `wagtail.contrib.settings.models.BaseSetting` model has been replaced by two new base models `BaseSiteSetting` and `BaseGenericSetting`, to accommodate settings that are shared across all sites. Existing setting models that inherit  `BaseSetting` should be updated to use `BaseSiteSetting` instead:
+
+```python
+from wagtail.contrib.settings.models import BaseSetting, register_setting
+
+@register_setting
+class SiteSpecificSocialMediaSettings(BaseSetting):
+    facebook = models.URLField()
+```
+
+should become
+
+```python
+from wagtail.contrib.settings.models import BaseSiteSetting, register_setting
+
+@register_setting
+class SiteSpecificSocialMediaSettings(BaseSiteSetting):
+    facebook = models.URLField()
+```