Document django-taggit 3.0.0 requirement for Django 4.1

Alternative to #9244, as per https://github.com/wagtail/wagtail/pull/9244#issuecomment-1255908946

docs/releases/4.0.md

@@ -13,7 +13,7 @@ depth: 1
 
 ### Django 4.1 support
 
-This release adds support for Django 4.1.
+This release adds support for Django 4.1. When upgrading, please note that the `django-taggit` library also needs to be updated to 3.0.0 or above.
 
 ### Global settings models