Clarify ByteBuf.duplicate() semantics.

Motivation:

From the javadocs of ByteBuf.duplicate() it is not clear if the reader and writer marks will be duplicated.

Modifications:

Add sentence to clarify that marks will not be duplicated.

Result:

Clear semantics.

buffer/src/main/java/io/netty/buffer/ByteBuf.java

@@ -1680,6 +1680,7 @@ public abstract class ByteBuf implements ReferenceCounted, Comparable<ByteBuf> {
      * This method is identical to {@code buf.slice(0, buf.capacity())}.
      * This method does not modify {@code readerIndex} or {@code writerIndex} of
      * this buffer.
+     * The reader and writer marks will not be duplicated.
      */
     public abstract ByteBuf duplicate();