[Youtube] Add some comments to mix

Author: XiangRongLin

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeParsingHelper.java

@@ -192,6 +192,12 @@ public static OffsetDateTime parseDateFrom(String textualUploadDate) throws Pars
         }
     }
 
+    /**
+     * Checks if the given playlist id is a mix (auto-generated playlist)
+     * Ids from a mix start with "RD"
+     * @param playlistId
+     * @return Whether given id belongs to a mix
+     */
     public static boolean isYoutubeMixId(String playlistId) {
         return playlistId.startsWith("RD");
     }

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/extractors/YoutubeMixPlaylistExtractor.java

@@ -18,6 +18,10 @@
 import org.schabi.newpipe.extractor.stream.StreamInfoItem;
 import org.schabi.newpipe.extractor.stream.StreamInfoItemsCollector;
 
+/**
+ * A YoutubePlaylistExtractor for a mix (auto-generated playlist).
+ * It handles urls in the format of "youtube.com/watch?v=videoId&list=playlistId"
+ */
 public class YoutubeMixPlaylistExtractor extends PlaylistExtractor {
 
   private Document doc;

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/linkHandler/YoutubePlaylistLinkHandlerFactory.java

@@ -71,6 +71,14 @@ public boolean onAcceptUrl(final String url) {
         return true;
     }
 
+    /**
+     * If it is a mix (auto-generated playlist) url, return a Linkhandler where the url is like
+     * youtube.com/watch?v=videoId&list=playlistId
+     * <p>Otherwise use super</p>
+     * @param url
+     * @return
+     * @throws ParsingException
+     */
     @Override
     public ListLinkHandler fromUrl(String url) throws ParsingException {
         try {