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/c16-proxy.md

@@ -17,6 +17,8 @@ auto res = cli.Get("/users");
 
 プロキシ経由でリクエストが送られます。HTTPSの場合はCONNECTメソッドでトンネルが張られるので、cpp-httplib側で特別な設定は要りません。
 
+プロキシがトンネルを拒否すると、リクエストは`Error::ProxyConnection`で失敗します。ただし407だけは例外で、レスポンスとして返ってきます。`res`はtruthyで`res->status`は407ですが、`res.error()`は`Error::ProxyConnection`のままです。
+
 ## プロキシに認証を設定する
 
 プロキシ自体が認証を要求する場合は、`set_proxy_basic_auth()`や`set_proxy_bearer_token_auth()`を使います。