2.15: Mention the removal of the strict_labels option in ReviewInput

The change was carried out in Icb9dfafea567e1e9c469811e830e3519b865700f.
Inclusion in the release notes appears necessary as per discussion on the
mailing list:

* by Jonas Bergler in
  https://groups.google.com/d/msg/repo-discuss/05s_xY123aE/mxKHnuLQAwAJ
* by Emmanual POT in
  https://groups.google.com/d/msg/repo-discuss/nMZo3HMgJSY/eXo3-vmYBQAJ

Change-Id: I8bf75ee0917d0599d1fa5e905fe7ce531720bf73
diff --git a/releases/2.15.md b/releases/2.15.md
index d7d81f7..4f233c1 100644
--- a/releases/2.15.md
+++ b/releases/2.15.md
@@ -137,6 +137,20 @@
 refer to the new `gerrit` user, so admins might need to tweak them to refer to
 `gerrit2` instead.
 
+### Strict validation on labels
+
+Posting a review including a vote on any labels that does not exist or is
+outside of the permitted range based on the ACLs, will now always result in a
+failure of the entire operation. Previously, a "strict labels" option set to
+disabled would allow posting these reviews and votes to such labels would be
+ignored silently. This option has now been removed from the
+[REST API's ReviewInput](#rest-api) as well as the
+[SSH review command](https://gerrit-documentation.storage.googleapis.com/Documentation/2.15/cmd-review.html).
+
+CI systems that rely on the non-strict behaviour of earlier Gerrit versions may
+require additional configuration to avoid casting votes to labels which are
+invalid for the change.
+
 ## New Features
 
 ### PolyGerrit UI
@@ -261,6 +275,12 @@
 * [ChangeInfo](https://gerrit-documentation.storage.googleapis.com/Documentation/2.15/rest-api-changes.html#change-info)
   now contains the submitter, for merged changes.
 
+* [ReviewInput](https://gerrit-documentation.storage.googleapis.com/Documentation/2.15/rest-api-changes.html#review-input)
+  no longer offers the `strict_labels` option. It will behave as enabled with
+  earlier versions of Gerrit. The entire operation will now fail if any of the
+  labels used are not within the user's permitted range based on ACLs or if the
+  label is not configured for the project/branch.
+
 #### Groups
 
 * [GroupInfo](https://gerrit-documentation.storage.googleapis.com/Documentation/2.15/rest-api-groups.html#group-info)