Refs #29260 -- Mentionned the behavior change of save for pk defaults…

… in the release notes.

docs/releases/3.0.txt

@@ -372,6 +372,22 @@ Tests
 Backwards incompatible changes in 3.0
 =====================================
 
+``Model.save`` on models when providing a default for the primary key
+---------------------------------------------------------------------
+
+Before Django 3.0 ``Model().save()`` would always attempt to find a row
+when a default value for the primary key was provided and proceed to perform
+either an ``INSERT`` or an ``UPDATE`` based on whether or not the row exists.
+
+Since Django 3.0 a single ``INSERT`` query is attempted which makes calling
+``Model().save()`` while providing a default primary key value equivalent to
+passing ``force_insert=True`` to ``save`. That makes attempts to use a new
+``Model`` instance to update an existing row result in an ``IntegrityError``.
+
+In order to update an existing model for a specific primary key value use the
+``get_or_update`` method defined on the model manager instead.
+
+
 Database backend API
 --------------------