doc: Improve doc for Http2 headers object

Add more details regarding processing and data type of incoming
headers in Http2.

doc/api/http2.md

@@ -2118,6 +2118,23 @@ means that normal JavaScript object methods such as
 `Object.prototype.toString()` and `Object.prototype.hasOwnProperty()` will
 not work.
 
+There is following processing done in 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
 const http2 = require('http2');
 const server = http2.createServer();