Update HeadlessJSAndroid.md explaining parameters

Summary:
HeadlessJsTaskConfig parameters can be unclear, so I added one more parameter that was left set by default and the respective comments.
Closes https://github.com/facebook/react-native/pull/16054

Differential Revision: D5881373

Pulled By: hramos

fbshipit-source-id: 3d271f73921417d82e3a2c7a1aaabe92aef7ec69

docs/HeadlessJSAndroid.md

@@ -43,7 +43,9 @@ public class MyTaskService extends HeadlessJsTaskService {
       return new HeadlessJsTaskConfig(
           "SomeTaskName",
           Arguments.fromBundle(extras),
-          5000);
+          5000, // timeout for the task
+          false // optional: defines whether or not  the task is allowed in foreground. Default is false
+        );
     }
     return null;
   }