doc: add authority and scheme psuedo headers

This pull request adds the request psuedo headers authority
and scheme to the http2 documentation

PR-URL: https://github.com/nodejs/node/pull/24777
Fixes: https://github.com/nodejs/node/issues/23825


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>