doc: add Http2Session.connecting property

PR-URL: #19842
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>

doc/api/http2.md

@@ -320,6 +320,17 @@ added: v9.4.0
 Will be `true` if this `Http2Session` instance has been closed, otherwise
 `false`.
 
+#### http2session.connecting
+<!-- YAML
+added: REPLACEME
+-->
+
+* {boolean}
+
+Will be `true` if this `Http2Session` instance is still connecting, will be set
+to `false` before emitting `connect` event and/or calling the `http2.connect`
+callback.
+
 #### http2session.destroy([error,][code])
 <!-- YAML
 added: v8.4.0