Update JavaDocs for comment replies continuation.

Co-authored-by: Stypox <stypox@pm.me>

Author: FireMasterK

extractor/src/main/java/org/schabi/newpipe/extractor/comments/CommentsInfoItemExtractor.java

@@ -111,7 +111,7 @@ default int getStreamPosition() throws ParsingException {
 
     /**
      * The continuation page which is used to get comment replies from.
-     * @return the continuation Page for the replies
+     * @return the continuation Page for the replies, or null if replies are not supported
      */
     @Nullable
     default Page getReplies() throws ParsingException {