Fixed #32675 -- Doc'd that autodector changes might cause generation of no-op migrations.

Follow up to aa4acc164d.

docs/releases/4.0.txt

@@ -392,6 +392,10 @@ Miscellaneous
 * Tests that fail to load, for example due to syntax errors, now always match
   when using :option:`test --tag`.
 
+* The migrations autodetector now uses model states instead of model classes.
+  As a side-effect ``makemigrations`` might generate no-op ``AlterField``
+  operations for ``ForeignKey`` fields in some cases.
+
 .. _deprecated-features-4.0:
 
 Features deprecated in 4.0