Add some java doc

https://github.com/stefan-niedermann/nextcloud-deck/issues/208

src/main/java/com/nextcloud/client/integration/deck/DeckApi.java

@@ -1,7 +1,6 @@
 package com.nextcloud.client.integration.deck;
 
 import android.app.PendingIntent;
-import android.content.Intent;
 
 import com.nextcloud.client.account.User;
 import com.nextcloud.java.util.Optional;
@@ -9,8 +8,20 @@ import com.owncloud.android.lib.resources.notifications.models.Notification;
 
 import androidx.annotation.NonNull;
 
+/**
+ * This API is for an integration with the <a href="https://github.com/stefan-niedermann/nextcloud-deck">Nextcloud
+ * Deck</a> app for android.
+ */
 public interface DeckApi {
 
+    /**
+     * Creates a PendingIntent that can be used in a NotificationBuilder to start the Deck app
+     *
+     * @param notification Notification object that should be processed
+     * @param user         The user that is affected by the notification
+     * @return Optional with a PendingIntent or an empty Optional if the notification is not from the
+     * <a href="https://apps.nextcloud.com/apps/deck">Deck server app</a>.
+     */
     @NonNull
     Optional<PendingIntent> createForwardToDeckActionIntent(@NonNull final Notification notification, @NonNull final User user);
 }