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/c18-ssl-errors.md

@@ -17,14 +17,17 @@ if (!res) {
   std::cerr << "error: " << httplib::to_string(err) << std::endl;
 
   if (err == httplib::Error::SSLConnection ||
-      err == httplib::Error::SSLServerVerification) {
+      err == httplib::Error::SSLServerVerification ||
+      err == httplib::Error::Read || err == httplib::Error::Write) {
     std::cerr << "ssl_error: " << res.ssl_error() << std::endl;
     std::cerr << "ssl_backend_error: " << res.ssl_backend_error() << std::endl;
   }
 }
 ```
 
-`ssl_error()` is a backend-independent TLS error category: an `httplib::tls::ErrorCode` cast to `int`. `ssl_backend_error()` gives you the backend's own error value. With OpenSSL that is `ERR_get_error()` when the handshake failed, and the verify result (`X509_V_ERR_*`) when certificate verification failed.
+`ssl_error()` is a backend-independent TLS error category: an `httplib::tls::ErrorCode` cast to `int`. `ssl_backend_error()` gives you the backend's own error value. With OpenSSL that is `ERR_get_error()` when the handshake or a later read or write failed, and the verify result (`X509_V_ERR_*`) when certificate verification failed.
+
+A request that fails after the handshake reports `Error::Read` or `Error::Write`. Even then, `ssl_error()` and `ssl_backend_error()` carry the TLS error behind it. For example, if the server requires a client certificate you didn't send, OpenSSL's error reads "tlsv13 alert certificate required". Both values are reset for each request and are `0` when no TLS error was involved.
 
 ## Format OpenSSL errors as strings