Fix README and docs pages that no longer match the code

Bring the READMEs and the docs pages (en and ja) in line with the
current header:

- Fix the client sample that does not compile, the Transfer-Encoding
  statement for default headers, the verifier callback comment, the
  thread pool spawn rule, the sanitize_filename description, and the
  inverted Accept-Encoding sentence.
- Describe recent behaviour: TLS errors behind Read/Write failures,
  Host validation, Connection: close for a body without length,
  content_reader rules, refused CONNECT, WebSocketClient proxies,
  CPPHTTPLIB_INTERIM_RESPONSE_MAX_COUNT, the SSE event size limit, and
  the payload limit of the Stream API.
- Complete the API references of WebSocket, WebSocketClient and the
  stream:: helpers, and the SSE reconnect rules.
- Fix cookbook pages: keychain loading on every backend, received Close
  status, a multipart sample that trusted the client's file name, an
  echo sample that sent binary as text, SNI, pre-request handler scope,
  stop() with WebSocket handlers, and the thread pool notes.
- Restore the mutex in the LLM app's Chapter 5 code.

docs-src/pages/en/cookbook/w05-websocket-tls.md

@@ -26,6 +26,8 @@ if (ws.connect()) {
 
 To disable certificate verification entirely, use `enable_server_certificate_verification(false)`. For details on that behavior, see [T02. Control SSL Certificate Verification](../t02-cert-verification).
 
+`WebSocketClient` also has `set_server_certificate_verifier()` and `set_session_verifier()` for custom checks, plus the proxy setters (`set_proxy()`, `set_proxy_basic_auth()`, `set_proxy_bearer_token_auth()`, `set_proxy_digest_auth()`, `set_no_proxy()`). CA certificates are loaded on the first `connect()`, so set them before that.
+
 ## Presenting a client certificate (mTLS)
 
 `ws::WebSocketClient` has a constructor overload that takes a `PemMemory` struct, letting `wss://` connections present a client certificate.