Update TESTING.asciidoc

Co-authored-by: Jordan Powers <[email protected]>

Author: pabloem

TESTING.asciidoc

@@ -685,7 +685,10 @@ There are multiple base classes for tests:
     These allow you to test functionality that is not exposed via the REST API, or for verifying a certain internal state.
     Additionally, you can easily simulate tricky distributed setups that are difficult to do in REST tests.
     If you only need to start one node, use `ESSingleNodeTestCase` instead, which is a much lighter test setup.
-* **`ESRestTestCase`**: This should be the first choice for writing integration tests. These tests run in a much more realistic setup, and rely on REST APIs to talk to ES.
+* **`ESRestTestCase`**: An integration test that interacts with an external
+  cluster via the REST API. This is used for Java based REST tests. This should
+  be the first choice for writing integration tests as these tests run in a much more
+  realistic setup.
 * **`ESClientYamlSuiteTestCase` **: A subclass of `ESRestTestCase` used to run YAML based REST tests.
 
 === Good practices