Update Plugin docs to reflect that all other interfaces are optional (cucumber#1431)

Author: mpkorstanje

core/src/main/java/cucumber/api/Plugin.java

@@ -10,12 +10,6 @@
  * A plugin can be added to the runtime to listen in on step definition, summary printing and test
  * execution.
  * <p>
- * Plugins must implement one of the following interfaces:
- * <ul>
- * <li>{@link cucumber.api.StepDefinitionReporter}</li>
- * <li>{@link cucumber.api.SummaryPrinter}</li>
- * </ul>
- * <p>
  * Plugins are added to the runtime from the command line or @{@link CucumberOptions} and may be provided with a
  * parameter. To accept this parameter the plugin must have a public constructor that accepts one of the following
  * arguments:
@@ -34,6 +28,8 @@
  * <li>{@link cucumber.api.formatter.ColorAware}</li>
  * <li>{@link cucumber.api.formatter.StrictAware}</li>
  * <li>{@link cucumber.api.event.EventListener}</li>
+ * <li>{@link cucumber.api.StepDefinitionReporter}</li>
+ * <li>{@link cucumber.api.SummaryPrinter}</li>
  * </ul>
  */
 public interface Plugin {