Document that clients must be kept referenced to avoid being shut down when garbage collected

[jnbdz]

Motivation

HttpClient, NetClient and WebSocketClient are lightweight proxies (CleanableHttpClient, CleanableNetClient, CleanableWebSocketClient) that are shut down when they are garbage collected. This is intentional, but it is not mentioned anywhere in the documentation, and users who create a client inline without keeping a reference to it regularly observe connections closed unexpectedly, requests refused with Client is closed, or WebSocket handshakes aborted, and report it as a bug: #5839 (WebSocket handshake), #5916 and #5987 (TCP / unpooled HTTP connections), #5577 (Pool closed). Even the test suite has been bitten (#5853, #6317).

Changes

Docs only.

• http.adoc, "Creating an HTTP client": IMPORTANT admonition explaining that a client must stay referenced for as long as it is used, that an unreferenced client is shut down when garbage collected as if HttpClient#shutdown() had been called (new requests refused, connections closed), the failures this surfaces as, and where to keep the reference.
• http.adoc, "WebSockets on the client": the same note, mentioning that WebSockets with a handshake in progress are closed as well.
• tcp.adoc, "Creating a TCP client": the same note pointing at NetClient#shutdown() and the existing "TCP graceful shut down" section (sockets without a shutdown handler are closed immediately).

While there, in the WebSocket client section:

• fixed two truncated sentences (e Vert.x ... → The Vert.x ..., en connecting ... → When connecting ...) and stray leading spaces;
• the clientWebSocketConfigurationAndConnect example block was indented by one space, so it was rendered as a literal paragraph rather than a source block; it now renders as [source,java].

Verified with mvn compile (docgen): the generated http.adoc / tcp.adoc contain the admonitions with resolved API links and the fixed source block.

[tsegismont]

Looks good to me, thanks