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.
Cookbook body links referenced sibling pages with a bare slug
(e.g. `c14-keep-alive`). Under the pretty-URL layout each page lives
in its own directory, so these resolve against the page's own
directory and 404. Prefix them with `../` to match the convention
already used in the tour and llm-app sections.
Verified clean with `docs-gen check`.
yhirose