Document ticket/asset display changes

docs/UPGRADING-6.0

@@ -83,6 +83,43 @@ fields using the C<%CustomFieldGroupings> option.
 
 =item *
 
+Now you can customize ticket/asset display pages via L<RT_Config/%WebDisplay>
+and some callbacks have been removed because of the change. If you use these
+callbacks, you will need to migrate it to a widget and add it to
+L<RT_Config/%WebDisplay> accordingly. To make RT upgrade easier, we have ported
+some of them so they can still work in RT 6.0, but it's strongly recommended to
+migrate all of them now.
+
+Affected callbacks include:
+
+=over
+
+=item /Ticket/Display.html BeforeShowSummary
+
+You can switch to C<BeforeWidget> in C</Elements/ShowWidgetSection>.
+
+=item /Ticket/Display.html BeforeShowHistory
+
+It's been ported to C</Ticket/Widget/History>, but C<Transactions> and
+C<Attachments> arguments are absent. You can switch to C<BeforeWidget>.
+
+=item /Asset/Display.html AfterShowSummary
+
+You can switch to C<AfterWidget> in C</Elements/ShowWidgetSection>.
+
+=item /Asset/Display.html AfterShowHistory
+
+It's been ported to C</Asset/Widget/History>, you can switch to C<AfterWidget>.
+
+=back
+
+Besides, C</Ticket/Elements/ShowSummary> and C</Asset/Elements/ShowSummary> are
+not called on display pages any more. If you have a customized version, you
+will need to customize corresponding widgets like "/Ticket/Widgets/Basics",
+"/Asset/Widgets/Basics", etc.
+
+=item *
+
 The following dependent libraries are updated to the new versions listed
 below in RT 6: