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/ja/cookbook/c19-client-logger.md

@@ -35,7 +35,7 @@ cli.set_error_logger([](const httplib::Error &err, const httplib::Request *req)
 });
 ```
 
-第2引数の`req`はヌルポインタのこともあります。リクエストを組み立てる前の段階で失敗した場合です。使う前に必ずヌルチェックしてください。
+第2引数の`req`はヌルポインタのこともあります。TLSハンドシェイクやCA証明書の読み込み、プロキシのトンネル確立など、接続の準備中にエラーが起きた場合です。使う前に必ずヌルチェックしてください。
 
 ## 両方を組み合わせる