Updated custom-keyed-values documentation

Release-Notes: skip
Change-Id: I8fa6befb54c950c129dd52df68f7e686874caddc

Documentation/rest-api-changes.txt

diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 844f4c6..8aa5c7f 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -2916,6 +2916,11 @@
 The custom keyed values to add or remove must be provided in the request body
 inside a link:#custom-keyed-values-input[CustomKeyedValuesInput] entity.
 
+Note that custom keyed values are expected to be small in both key and value.
+A typical use-case would be storing the ID to some external system, in which
+case the key would be something unique to that system and the value would be
+the ID.
+
 .Request
 ----
   POST /changes/myProject~master~I8473b95934b5732ac55d26311a706c9c2bde9940/custom_keyed_values HTTP/1.0