Clarify writability in Netty4HttpPipeliningHandler (#91982)

TIL we don't really mean that the physical channel is writable in these
loops, so this commit adds a couple of comments to record that lesson.

Author: DaveCTurner

modules/transport-netty4/src/main/java/org/elasticsearch/http/netty4/Netty4HttpPipeliningHandler.java

@@ -216,7 +216,8 @@ private void doWrite(ChannelHandlerContext ctx, Netty4ChunkedHttpResponse readyR
         combiner.add((Future<Void>) first);
         currentChunkedWrite = new ChunkedWrite(combiner, promise, readyResponse);
         if (enqueueWrite(ctx, readyResponse, first)) {
-            // we were able to write out the first chunk directly, try writing out subsequent chunks until the channel becomes unwritable
+            // We were able to write out the first chunk directly, try writing out subsequent chunks until the channel becomes unwritable.
+            // NB "writable" means there's space in the downstream ChannelOutboundBuffer, we aren't trying to saturate the physical channel.
             while (ctx.channel().isWritable()) {
                 if (writeChunk(ctx, combiner, readyResponse.body())) {
                     finishChunkedWrite();
@@ -280,6 +281,7 @@ private boolean doFlush(ChannelHandlerContext ctx) throws IOException {
             return false;
         }
         while (channel.isWritable()) {
+            // NB "writable" means there's space in the downstream ChannelOutboundBuffer, we aren't trying to saturate the physical channel.
             WriteOperation currentWrite = queuedWrites.poll();
             if (currentWrite == null) {
                 doWriteQueued(ctx);