Add UI extension point for header bar in change screen

E.g. plugins may use this extension point to display a custom status
in the header bar on the change screen.

The cookbook plugin implements an example for this extension point.

Change-Id: I0e2e66c1c38e6e25efe749edb12cfe4e8051faeb
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
diff --git a/Documentation/dev-plugins.txt b/Documentation/dev-plugins.txt
index aa6cd35..123be8c 100644
--- a/Documentation/dev-plugins.txt
+++ b/Documentation/dev-plugins.txt
@@ -971,6 +971,11 @@
 panels with custom controls:
 
 * Change Screen:
+** `GerritUiExtensionPoint.CHANGE_SCREEN_HEADER`:
++
+Panel will be shown in the header bar to the right of the change
+status.
+
 ** `GerritUiExtensionPoint.CHANGE_SCREEN_BELOW_CHANGE_INFO_BLOCK`:
 +
 Panel will be shown below the change info block.