Add comment on function with config dserialization randomization

Author: ivancea

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/scalar/AbstractConfigurationFunctionTestCase.java

@@ -36,6 +36,7 @@ public void testSerializationWithConfiguration() {
 
         Configuration differentConfig = randomValueOtherThan(
             config,
+            // The source must match the original (static) one, as function source serialization depends on it
             () -> randomConfiguration(testCase.getSource().text(), randomTables())
         );