Docs: changes /index endpoint requires administrateServer capability

The documentation of the /index REST API endpoint was added
with commit 4e38b97171ed4851395655435ab88d7d93da6bee but omitted
to mention that the caller must be a member of a group that is
granted the administrateServer capability.

Change-Id: Ic71dcd887e2f7ec3ea3194102aa8cd4f6c539398

Documentation/rest-api-changes.txt

diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 4d65a83..1ce7d6c 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -1078,6 +1078,10 @@
 
 Adds or updates the change in the secondary index.
 
+The caller must be a member of a group that is granted the
+link:access-control.html#capability_administrateServer[
+Administrate Server] capability.
+
 .Request
 ----
   POST /changes/myProject~master~I8473b95934b5732ac55d26311a706c9c2bde9940/index HTTP/1.0