http2: doc and fixes to the Compatibility API

Backport-PR-URL: #14813
Backport-Reviewed-By: Anna Henningsen <[email protected]>
Backport-Reviewed-By: Timothy Gu <[email protected]>

PR-URL: #14239
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>

Author: mcollina