[TEST] clarified skip features usage

Author: javanna

rest-api-spec/test/README.asciidoc

@@ -88,6 +88,8 @@ The skip section can also be used to list new features that need to be
 supported in order to run a test. This way the up-to-date runners will
 run the test, while the ones that don't support the feature yet can
 temporarily skip it, and avoid having lots of test failures in the meantime.
+Once all runners have implemented the feature, it can be declared supported
+by default, thus the related skip sections can be removed from the tests.
 
 ....
     "Parent":

src/test/java/org/elasticsearch/test/rest/support/Features.java

@@ -27,6 +27,9 @@
  * Allows to register additional features supported by the tests runner.
  * This way any runner can add extra features and use proper skip sections to avoid
  * breaking others runners till they have implemented the new feature as well.
+ *
+ * Once all runners have implemented the feature, it can be removed from the list
+ * and the related skip sections can be removed from the tests as well.
  */
 public final class Features {