Update comments

extractor/src/main/java/org/schabi/newpipe/extractor/stream/Frameset.java

@@ -22,6 +22,9 @@ public final class Frameset {
 		this.framesPerPageY = framesPerPageY;
 	}
 
+	/**
+	 * @return list of urls to images with frames
+	 */
 	public List<String> getUrls() {
 		return urls;
 	}

extractor/src/main/java/org/schabi/newpipe/extractor/stream/StreamExtractor.java

@@ -259,8 +259,10 @@ public abstract class StreamExtractor extends Extractor {
     public abstract StreamInfoItemsCollector getRelatedStreams() throws IOException, ExtractionException;
 
     /**
-     * Should return a list of frames
-     * @return
+     * Should return a list of Frameset object that contains preview of stream frames
+     * @return list of preview frames or empty list if frames preview is not supported or not found for specified stream
+     * @throws IOException
+     * @throws ExtractionException
      */
     @Nonnull
     public List<Frameset> getFrames() throws IOException, ExtractionException {