Documentation update for netsocket/websocket writeHandlers

Follows-up on #4625

Signed-off-by: Thomas Segismont <tsegismont@gmail.com>

src/main/asciidoc/http.adoc

@@ -1896,12 +1896,14 @@ used with binary frames that are no split over multiple frames.
 
 ==== Event bus handlers
 
-Every WebSocket automatically registers two handler on the event bus, and when any data are received in this handler,
-it writes them to itself. Those are local subscriptions not routed on the cluster.
+Every WebSocket can register two handlers on the event bus, and when any data are received in these handlers,
+it writes the data to itself. Those are local subscriptions, not reachable from other clustered nodes.
 
 This enables you to write data to a WebSocket which is potentially in a completely different verticle sending data
 to the address of that handler.
 
+This feature is disabled by default, however you can enable it using {@link io.vertx.core.http.HttpServerOptions#setRegisterWebSocketWriteHandlers} or {@link io.vertx.core.http.WebSocketConnectOptions#setRegisterWriteHandlers}.
+
 The addresses of the handlers are given by {@link io.vertx.core.http.WebSocket#binaryHandlerID()} and
 {@link io.vertx.core.http.WebSocket#textHandlerID()}.
 

src/main/asciidoc/net.adoc

@@ -124,12 +124,14 @@ exceptions that happens before the connection is passed to the {@link io.vertx.c
 
 === Event bus write handler
 
-Every socket automatically registers a handler on the event bus, and when any buffers are received in this handler,
-it writes them to itself. Those are local subscriptions not routed on the cluster.
+Every socket can register a handler on the event bus, and when any buffers are received in this handler,
+it writes them to itself. Those are local subscriptions, not reachable from other clustered nodes.
 
 This enables you to write data to a socket which is potentially in a completely different verticle by sending the buffer to the address of that handler.
 
-The address of the handler is given by {@link io.vertx.core.net.NetSocket#writeHandlerID()}
+This feature is disabled by default, however you can enable it using {@link io.vertx.core.net.NetServerOptions#setRegisterWriteHandler} or {@link io.vertx.core.net.NetClientOptions#setRegisterWriteHandler}.
+
+The address of the handler is given by {@link io.vertx.core.net.NetSocket#writeHandlerID()}.
 
 === Local and remote addresses