Add comment to explain the extra test check for FIPS

Author: ywangd

modules/transport-netty4/src/javaRestTest/java/org/elasticsearch/rest/Netty4HeadBodyIsEmptyIT.java

@@ -128,6 +128,8 @@ public void testTemplateExists() throws IOException {
             builder.endObject();
 
             Request request = new Request("PUT", "/_template/template");
+            // The warnings only need to be checked in FIPS mode because we run default distribution for FIPS,
+            // while the integ-test distribution is used otherwise.
             if (inFipsJvm()) {
                 request.setOptions(expectWarnings(
                     "legacy template [template] has index patterns [*] matching patterns from existing composable templates " +