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/tour/09-whats-next.md

@@ -21,7 +21,9 @@ if (result) {
 }
 ```
 
-`Get()`に`content_receiver`コールバックを渡す方法もあります。こちらはKeep-Aliveと併用できます。
+なお、`stream::Get()`で読めるボディの大きさは、クライアントの`set_payload_max_length()`の上限（デフォルトは100MB）までです。もっと大きなファイルをダウンロードするなら、上限を引き上げるか、0を指定して無制限にしてください。
+
+`Get()`に`content_receiver`コールバックを渡す方法もあります。こちらはKeep-Aliveと併用でき、`set_payload_max_length()`で上限を設定しない限りサイズの制限もありません。
 
 ```cpp
 httplib::Client cli("http://localhost:8080");