doc: remove http cookie ua from list of buffers

as they are available via library using the tx
- tx:request_header("Cookie")
- tx:response_header("Cookie")
- tx:request_header("User-Agent")

doc/userguide/rules/lua-detection.rst

@@ -50,13 +50,10 @@ inspection. Currently the following are available:
 * http.request_line
 * http.request_headers
 * http.request_headers.raw
-* http.request_cookie
-* http.request_user_agent
 * http.request_body
 * http.response_headers
 * http.response_headers.raw
 * http.response_body
-* http.response_cookie
 
 All the HTTP buffers have a limitation: only one can be inspected by a
 script at a time.