doc: Improve doc for Http2 headers object

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

PR-URL: https://github.com/nodejs/node/pull/21296


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>