Clarify penalty performance for alternative change identifiers

Release-Notes: skip
Change-Id: I3c84998aca7e5f09e1e85c616bc70ce044caa6fd

Documentation/rest-api-changes.txt

diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 72ee0b9..3974428 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -6262,7 +6262,13 @@
 Identifier that uniquely identifies one change. It contains the URL-encoded
 project name as well as the change number: "'$$<project>~<numericId>$$'"
 
-Gerrit also supports the following identifiers:
+==== Alternative identifiers
+Gerrit also supports an array of other change identifiers.
+
+[NOTE]
+Using these identifiers comes with a performance penalty as they
+require additional lookups to be translated to the
+"'$$<project>~<numericId>$$'" identifier.
 
 * an ID of the change in the format "'$$<project>~<branch>~<Change-Id>$$'",
   where for the branch the `refs/heads/` prefix can be omitted