docs: add marker explanation (#2945)

marker/src/main/java/App.java

@@ -22,18 +22,18 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  * THE SOFTWARE.
  */
-import org.slf4j.Logger;
+
 import org.slf4j.LoggerFactory;
 
 /**
  * Created by Alexis on 28-Apr-17. With Marker interface idea is to make empty interface and extend
  * it. Basically it is just to identify the special objects from normal objects. Like in case of
  * serialization , objects that need to be serialized must implement serializable interface (it is
- * empty interface) and down the line writeObject() method must be checking if it is a instance of
+ * empty interface) and down the line writeObject() method must be checking if it is an instance of
  * serializable or not.
  *
  * <p>Marker interface vs annotation Marker interfaces and marker annotations both have their uses,
- * neither of them is obsolete or always better then the other one. If you want to define a type
+ * neither of them is obsolete or always better than the other one. If you want to define a type
  * that does not have any new methods associated with it, a marker interface is the way to go. If
  * you want to mark program elements other than classes and interfaces, to allow for the possibility
  * of adding more information to the marker in the future, or to fit the marker into a framework