Clarify that change_message_id is only available for change comments

This field is only available for change comments and not for revision
comments.

Change-Id: I105f85f37d29fec2369a426b435fdea050d47d4a

Documentation/rest-api-changes.txt

diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 6212b01..fbad212 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -6539,9 +6539,9 @@
 resolution of a comment thread is stored in the last comment in that thread
 chronologically.
 |`change_message_id` |optional|
-Available with published comments. Contains the
-link:rest-api-changes.html#change-message-info[id] of the change message
-that this comment is linked to.
+Available with the link:#list-change-comments[list change comments] endpoint.
+Contains the link:rest-api-changes.html#change-message-info[id] of the change
+message that this comment is linked to.
 |`commit_id` |optional|
 Hex commit SHA1 (40 characters string) of the commit of the patchset to which
 this comment applies.