docs: Backend Protocol Selection

> we chatted about this the meeting today, the `http2` setting in BTP works as expected, its meant to shape setting if the protocol selected is http2, but not necessarily set it, this field is handy when Gateway level BTPs can be used to set sane default for app owners who may choose to opt into http2
>
> we can def do a better job to explain protocol selection in the docs, creating a new issue to track it

_Originally posted by @arkodg in [#8648](https://github.com/envoyproxy/gateway/issues/8648#issuecomment-4261231607)_