Document test instance semantics for TestInstancePostProcessors

Issue: #419

junit-jupiter-api/src/main/java/org/junit/jupiter/api/extension/TestInstancePostProcessor.java

@@ -25,13 +25,20 @@
  * <p>Implementations must provide a no-args constructor.
  *
  * @since 5.0
+ * @see #postProcessTestInstance(Object, ExtensionContext)
  */
 @API(Experimental)
 public interface TestInstancePostProcessor extends Extension {
 
 	/**
 	 * Callback for post-processing the supplied test instance.
 	 *
+	 * <p><strong>Note</strong>: the {@code ExtensionContext} supplied to a
+	 * {@code TestInstancePostProcessor} will always return an empty
+	 * {@link java.util.Optional} value from {@link ExtensionContext#getTestInstance()
+	 * getTestInstance()}. A {@code TestInstancePostProcessor} should therefore
+	 * only attempt to process the supplied {@code testInstance}.
+	 *
 	 * @param testInstance the instance to post-process; never {@code null}
 	 * @param context the current extension context; never {@code null}
 	 */