Clarify that change-number is valid change identifier if unique

Release-Notes: skip
Change-Id: If7fcfd67cf85612ec8940c2942a1b88a2f362bc2
diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index da3018b..72ee0b9 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -6269,7 +6269,7 @@
   ("$$myProject~master~I8473b95934b5732ac55d26311a706c9c2bde9940$$")
 * a Change-Id if it uniquely identifies one change
   ("I8473b95934b5732ac55d26311a706c9c2bde9940")
-* a numeric change ID ("4247")
+* a numeric change ID if it uniquely identifies one change ("4247")
 
 [[change-message-id]]
 === \{change-message-id\}