Update modal docs

- Relates #825

spring-shell-core/src/main/java/org/springframework/shell/component/view/control/ViewService.java

@@ -24,9 +24,25 @@ import org.springframework.lang.Nullable;
  */
 public interface ViewService {
 
+	/**
+	 * Gets a current modal view.
+	 *
+	 * @return current modal view
+	 */
+	@Nullable
 	View getModal();
 
-	void setModal(View view);
+	/**
+	 * Sets a new modal view. Setting modal to {@code null} clears existing modal.
+	 *
+	 * @param view a view to use as modal
+	 */
+	void setModal(@Nullable View view);
 
+	/**
+	 * Sets a view to be focused. Setting focus to {@code null} clears focused view.
+	 *
+	 * @param view a view to be focused
+	 */
 	void setFocus(@Nullable View view);
 }