Fix and clearify javadocs

Motivation:

ByteToMessageDecoder and ReplayingDecoder have incorrect javadocs in some places.

Modifications:

Fix incorrect javadocs for both classes.

Result:

Correct javadocs for both classes

codec/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java

@@ -284,13 +284,12 @@ public abstract class ByteToMessageDecoder extends ChannelHandlerAdapter {
 
     /**
      * Decode the from one {@link ByteBuf} to an other. This method will be called till either the input
-     * {@link ByteBuf} has nothing to read anymore, till nothing was read from the input {@link ByteBuf} or till
-     * this method returns {@code null}.
+     * {@link ByteBuf} has nothing to read when return from this method or till nothing was read from the input
+     * {@link ByteBuf}.
      *
      * @param ctx           the {@link ChannelHandlerContext} which this {@link ByteToMessageDecoder} belongs to
      * @param in            the {@link ByteBuf} from which to read data
      * @param out           the {@link List} to which decoded messages should be added
-
      * @throws Exception    is thrown if an error accour
      */
     protected abstract void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception;

codec/src/main/java/io/netty/handler/codec/ReplayingDecoder.java

@@ -240,7 +240,7 @@ import java.util.List;
  * public class FirstDecoder extends {@link ReplayingDecoder}<{@link Void}> {
  *
  *     {@code @Override}
- *     protected Object decode({@link ChannelHandlerContext} ctx,
+ *     protected void decode({@link ChannelHandlerContext} ctx,
  *                             {@link ByteBuf} buf, List<Object> out) {
  *         ...
  *         // Decode the first message