Add release note for policy sample file update

The sample policy file was updated recently and this releasenote
explains the changes for operators.

A more narrowly scoped releasenote for a previous change along similar
lines has been removed since it is covered under this note.

Change-Id: I11bde778e9fe1f3a70d9fac213b40f05f07e7e47

releasenotes/notes/lock_policy-75bea372036acbd5.yaml

@@ -1,6 +0,0 @@
----
-upgrade:
-  - Default RBAC policy for lock operations has been modified to
-    admin_or_owner for the stable V2.0 API. Please understand to modify the
-    policy if you still keep to have anyone to lock an instance and you're
-    still using the stable API endpoint.

releasenotes/notes/policy-sample-defaults-changed-b5eea1daeb305251.yaml

@@ -0,0 +1,16 @@
+---
+other:
+
+  - The sample policy file shipped with Nova contained many policies set to
+    ""(allow all) which was not the proper default for many of those checks. It
+    was also a source of confusion as some people thought "" meant to use the
+    default rule. These empty policies have been updated to be explicit in all
+    cases.
+
+    Many of them were changed to match the default rule of "admin_or_owner"
+    which is a more restrictive policy check but does not change the
+    restrictiveness of the API calls overall because there are similar checks
+    in the database already.
+
+    This does not affect any existing deployment, just the sample file included
+    for use by new deployments.