Reveal Submission Id in ChangeInfo

As a follow-up for RevertSubmission (b81f56c469),
it is useful to reveal the submission_id to the
frontend so that the frontend can suggest better
commit message to the user.

Change-Id: Ic557e0c948bd5029478e5538785cf4b9943396bf

Documentation/rest-api-changes.txt

diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 4165f93..69a7641 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -6010,6 +6010,8 @@
 When present, change has been marked Ready at some point in time.
 |`revert_of`          |optional|
 The numeric Change-Id of the change that this change reverts.
+|`submission_id`      |optional|
+ID of the submission of this change. Only set if the status is `MERGED`.
 |==================================
 
 [[change-input]]