doc: include http2.md in all.md

Backport-PR-URL: #14813
Backport-Reviewed-By: Anna Henningsen <anna@addaleax.net>
Backport-Reviewed-By: Timothy Gu <timothygu99@gmail.com>

PR-URL: #14239
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: jasnell

doc/api/all.md

@@ -19,6 +19,7 @@
 @include fs
 @include globals
 @include http
+@include http2
 @include https
 @include inspector
 @include intl