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/s03-path-params.md

@@ -44,6 +44,8 @@ svr.Get(R"(/users/(\d+))", [](const httplib::Request &req, httplib::Response &re
 
 パターンに括弧を使うと、マッチした部分が`req.matches`に入ります。`req.matches[0]`はパス全体、`req.matches[1]`以降がキャプチャです。
 
+正規表現のルートは、`CPPHTTPLIB_REGEX_ROUTE_PATH_MAX_LENGTH`（デフォルトは256）より長いパスには適用されず、マッチしなかったものとして扱われます。長い入力で`std::regex`がスタックを使い果たさないための制限です。もっと長いパスを扱いたい場合は、このマクロに大きな値を定義してください。
+
 ## どちらを使うか
 
 - 単純なIDやスラッグなら`:name`でじゅうぶん。読みやすく、型が自明です