Add unit to maxContentLength message javadoc

Motivation:
Not knowing which unit is returned by the maxContentLength() of the Messageggregator when reading the Javadoc is annoying and can be a source of bugs.

Modifications:
Added the mention "in bytes"

Result:
Javadoc is clear.

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

@@ -135,7 +135,7 @@ public abstract class MessageAggregator<I, S, C extends ByteBufHolder, O extends
     protected abstract boolean isAggregated(I msg) throws Exception;
 
     /**
-     * Returns the maximum allowed length of the aggregated message.
+     * Returns the maximum allowed length of the aggregated message in bytes.
      */
     public final int maxContentLength() {
         return maxContentLength;