Document error field in ReviewResult

Signed-off-by: Edwin Kempin <ekempin@google.com>
Release-Notes: skip
Change-Id: I0fa5e94482c281544ea361b60d3d6d4399fcc801
diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 4354bb3..0c5f96f 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -8150,6 +8150,8 @@
 |`ready`                  |optional|
 If true, the change was moved from WIP to ready for review as a result of this
 action. Not set if false.
+|`error`                  |optional|
+Error message for non-200 responses.
 |============================
 
 [[reviewer-info]]