[DOCS] Update readme for testing x-pack code snippets (#30696)

CONTRIBUTING.md

@@ -175,6 +175,15 @@ Before submitting your changes, run the test suite to make sure that nothing is
 ./gradlew check
 ```
 
+If your changes affect only the documentation, run:
+
+```sh
+./gradlew -p docs check
+```
+For more information about testing code examples in the documentation, see 
+https://github.com/elastic/elasticsearch/blob/master/docs/README.asciidoc
+
+
 Contributing as part of a class
 -------------------------------
 In general Elasticsearch is happy to accept contributions that were created as

docs/README.asciidoc

@@ -8,6 +8,9 @@ CONSOLE" and "COPY AS CURL" in the documentation and are automatically tested
 by the command `gradle :docs:check`. To test just the docs from a single page,
 use e.g. `gradle :docs:check -Dtests.method=*rollover*`.
 
+NOTE: If you have an elasticsearch-extra folder alongside your elasticsearch 
+folder, you must temporarily rename it when you are testing 6.3 or later branches.
+
 By default each `// CONSOLE` snippet runs as its own isolated test. You can
 manipulate the test execution in the following ways: