doc: cookie is joined using '; '

document that incoming cookie headers are joined using '; '.

PR-URL: nodejs#24740
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/http.md

@@ -1598,6 +1598,7 @@ header name:
 `last-modified`, `location`, `max-forwards`, `proxy-authorization`, `referer`,
 `retry-after`, or `user-agent` are discarded.
 * `set-cookie` is always an array. Duplicates are added to the array.
+* For duplicate `cookie` headers, the values are joined together with '; '.
 * For all other headers, the values are joined together with ', '.
 
 ### message.httpVersion

doc/api/http2.md

@@ -2297,7 +2297,7 @@ For incoming headers:
 `upgrade-insecure-requests`, `user-agent` or `x-content-type-options` are
 discarded.
 * `set-cookie` is always an array. Duplicates are added to the array.
-* `cookie`: the values are joined together with '; '.
+* For duplicate `cookie` headers, the values are joined together with '; '.
 * For all other headers, the values are joined together with ', '.
 
 ```js