2.16: Add a "Differences from GWT" section

There's not much in this section, but there's no other place suitable
to mention things like the behavioral change reported in issue 10513.

Bug: Issue 10513
Change-Id: I312a3364c72a79dc4a45b339080efced3f992ede
diff --git a/pages/site/releases/2.16.md b/pages/site/releases/2.16.md
index 9087342..b880d9f 100644
--- a/pages/site/releases/2.16.md
+++ b/pages/site/releases/2.16.md
@@ -278,6 +278,14 @@
 - [Issue 9216](https://bugs.chromium.org/p/gerrit/issues/detail?id=9216)
   Close the groups editor after submitting
 
+#### Differences from GWT
+
+- Writing a cover message on a specific patch set is no longer possible.
+
+  The cover message is always added on the current patch set. This means
+  that any system that relies on notifications being sent for a specific
+  patch set will no longer work.
+
 ### New logging framework
 
 For logging Gerrit is now using [Flogger](https://github.com/google/flogger)