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/s03-path-params.md

@@ -44,6 +44,8 @@ svr.Get(R"(/users/(\d+))", [](const httplib::Request &req, httplib::Response &re
 
 Parentheses in the pattern become captures in `req.matches`. `req.matches[0]` is the full match; `req.matches[1]` onward are the captures.
 
+A regex route isn't applied to a path longer than `CPPHTTPLIB_REGEX_ROUTE_PATH_MAX_LENGTH` (256 by default); such a path is treated as no match. This keeps `std::regex` from overflowing the stack on long input. If your routes need longer paths, define the macro with a larger value.
+
 ## Which to use
 
 - For plain IDs or slugs, `:name` is enough — readable, and the shape is obvious