Updating docs for test classes (#134076)

* Updating docs for test classes

* comments

* Update TESTING.asciidoc

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

---------

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

Author: pabloem

TESTING.asciidoc

@@ -681,12 +681,15 @@ There are multiple base classes for tests:
   directly by unit tests.
 * **`ESSingleNodeTestCase`**: This test case sets up a cluster that has a
   single node.
-* **`ESIntegTestCase`**: An integration test case that creates a cluster that
-  might have multiple nodes.
-* **`ESRestTestCase`**: An integration tests that interacts with an external
-  cluster via the REST API. This is used for Java based REST tests.
-* **`ESClientYamlSuiteTestCase` **: A subclass of `ESRestTestCase` used to run
-  YAML based REST tests.
+* **`ESIntegTestCase`**: An internal integration test that starts nodes within the same JVM as the test.
+    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`**: 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