Add spec points around HTTP/2 support requirements for SDKs

[QuintinWillison]

See this Slack thread (internal) where...

@lmars stated:

we support HTTP/2 on our primary endpoints, but not on our fallback endpoints. We should document that

and then:

client side we need to handle HTTP/2 in the expected way (try and use it, but then fallback to HTTP/1.1 if the server doesn't indicate it supports HTTP/2)

and @mattheworiordan stated:

I did explicitly ask about this in https://github.com/ably/ideas/issues/309#issuecomment-1096763108, but didn’t get a response. I assumed it was covered, but perhaps shouldn’t have assumed that. I think we need to be explicit on our spec. Could you propose a PR to address this?

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-2793

[mattheworiordan]

Please note we need to also update our docs so that developers know our SDKs are HTTP/2 compatible.