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/s09-pre-routing.md

@@ -16,7 +16,7 @@ svr.set_pre_routing_handler(
   });
 ```
 
-The pre-routing handler runs **before routing**. It catches every request — including ones that don't match any handler.
+The pre-routing handler runs **before routing**. It catches every request — including ones that don't match any handler. The exception is a request rejected while its request line and headers are parsed (400 for a malformed request, 414 for an overlong URI, 416 for a malformed `Range`); the server answers those before this handler runs.
 
 The `HandlerResponse` return value is key: