doc: add authority and scheme psuedo headers

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

PR-URL: nodejs#24777
Fixes: nodejs#23825
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>

doc/api/http2.md

@@ -2654,6 +2654,16 @@ added: v10.1.0
 The `request.aborted` property will be `true` if the request has
 been aborted.
 
+#### request.authority
+<!-- YAML
+added: v8.4.0
+-->
+
+* {string}
+
+The request authority pseudo header field. It can also be accessed via
+`req.headers[':authority']`.
+
 #### request.destroy([error])
 <!-- YAML
 added: v8.4.0
@@ -2762,6 +2772,16 @@ added: v8.4.0
 The raw request/response trailer keys and values exactly as they were
 received. Only populated at the `'end'` event.
 
+#### request.scheme
+<!-- YAML
+added: v8.4.0
+-->
+
+* {string}
+
+The request scheme pseudo header field indicating the scheme
+portion of the target URL.
+
 #### request.setTimeout(msecs, callback)
 <!-- YAML
 added: v8.4.0