docs: cancellation reaches the proxy hop, and where ok() stands

The README example uses the jthread's own token. CHANGELOG now names the
proxies the stop reaches and the Socket and TlsSocket members that are new,
and both say that ok() does not look at cancelled, as it does not look at
bodyComplete.

Author: yspbwx2010

CHANGELOG.md

@@ -7,16 +7,20 @@ can abandon a request that is in flight.
 
 * When the token is stopped the call returns within about 50 ms with
   `HttpResponse::cancelled` set, and the connection is closed instead of being
-  returned to the pool. It covers connect, the proxy CONNECT exchange, the TLS
-  handshake, waiting for the response head, reading the body and the gaps
-  between streaming callbacks.
+  returned to the pool. It covers connect, the exchange with an HTTP, https or
+  SOCKS5 proxy, the TLS handshake, waiting for the response head, reading the
+  body and the gaps between streaming callbacks.
 * Before the status line arrives the response is `statusCode` 0 with
   `statusText` and `bodyError` `Cancelled`. After it, `statusCode` is the
   server's and `bodyError` is `cancelled`. A cancelled request is not retried on
-  a new connection and its redirect is not followed.
-* Not covered: name resolution, and a write that is blocked because the server
-  is not reading. Without a token nothing changes.
-* `proxy_connect` takes the token as a trailing default argument.
+  a new connection and its redirect is not followed. `ok()` does not look at
+  `cancelled`, as it does not look at `bodyComplete`.
+* Not covered: name resolution (`getaddrinfo`, and the lookup `socks5://` does
+  for the target), and a write that is blocked because the server is not
+  reading. Without a token nothing changes.
+* `proxy_connect` and `proxy_tunnel` take the token as a trailing default
+  argument. `Socket` and `TlsSocket` gain `set_stop`, and `Socket` gains
+  `stop_possible`.
 
 ## 0.3.3
 

README.md

@@ -53,16 +53,16 @@ another thread. The call returns within about 50 ms with `cancelled` set, and
 the connection is closed rather than pooled.
 
 ```cpp
-std::stop_source source;
-auto worker = std::jthread([&] { response = client.send(request, source.get_token()); });
+HttpResponse response;
+std::jthread worker([&](std::stop_token stop) { response = client.send(request, stop); });
 // elsewhere:
-source.request_stop();
+worker.request_stop();
 ```
 
 If the status line had not arrived, `statusCode` is 0 and `statusText` is
 `Cancelled`; otherwise they are the server's and `bodyError` is `cancelled`.
-Name resolution and a write blocked on a server that is not reading cannot be
-interrupted.
+As with `bodyComplete`, `ok()` does not look at `cancelled`. Name resolution
+and a write blocked on a server that is not reading cannot be interrupted.
 
 ### Configuration