Expose instanceId in GerritInfo

Change-Id: I1ae138cf98fa1bfac9f2ccfea63d4982194cec5b
diff --git a/Documentation/rest-api-config.txt b/Documentation/rest-api-config.txt
index 9764c8a..a8d9b3d 100644
--- a/Documentation/rest-api-config.txt
+++ b/Documentation/rest-api-config.txt
@@ -1839,6 +1839,8 @@
 Whether to enable the web UI for editing GPG keys.
 |`report_bug_url`    |optional|
 link:config-gerrit.html#gerrit.reportBugUrl[URL to report bugs].
+|`instance_id`       |optional|
+link:config-gerrit.html#gerrit.instanceId[Short identifier for this Gerrit installation].
 |=================================
 
 [[index-config-info]]