doc: Improve doc for Http2 headers object

Add more details regarding processing and data type of incoming headers in Http2. Backport-PR-URL: #22850 PR-URL: #21296 Reviewed-By: Anna Henningsen <[email protected]> Reviewed-By: James M Snell <[email protected]> Reviewed-By: Ujjwal Sharma <[email protected]>

Author: Flarna

doc/api/http2.md

@@ -2120,6 +2120,23 @@ prototype. This means that normal JavaScript object methods such as
 `Object.prototype.toString()` and `Object.prototype.hasOwnProperty()` will
 not work.
 
+For incoming headers:
+* The `:status` header is converted to `number`.
+* Duplicates of `:status`, `:method`, `:authority`, `:scheme`, `:path`,
+`age`, `authorization`, `access-control-allow-credentials`,
+`access-control-max-age`, `access-control-request-method`, `content-encoding`,
+`content-language`, `content-length`, `content-location`, `content-md5`,
+`content-range`, `content-type`, `date`, `dnt`, `etag`, `expires`, `from`,
+`if-match`, `if-modified-since`, `if-none-match`, `if-range`,
+`if-unmodified-since`, `last-modified`, `location`, `max-forwards`,
+`proxy-authorization`, `range`, `referer`,`retry-after`, `tk`,
+`upgrade-insecure-requests`, `user-agent` or `x-content-type-options` are
+discarded.
+*`set-cookie` is a string if present once or an array in case duplicates
+are present.
+*`cookie`: the values are joined together with ' '.
+* For all other headers, the values are joined together with ', '.
+
 ```js
 consthttp2=require('http2');
 constserver=http2.createServer();