Align documentation about HTTPRouteTimeouts.BackendRequest timeout scope

[rtribotte]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

This PR aligns the HTTPRouteTimeouts.BackendRequest timeout documentation.
It is stated in the GEP 1742 sequence diagram illustrating the timeouts boundaries that the HTTPRouteTimeouts.BackendRequest timeout runs until the responses headers are received:

gateway-api/geps/gep-1742/index.md

Lines 358 to 360 in f735045

	U->>P: Starts Response
	U->>P: Finishes Headers
	note right of P: timeouts.backendRequest end time

The documentation for the CRD was mentioning a timeout running until the end of the response, which is not consistent and this PR aims to align expectations.

On the side, in the GEP 1742, the Traefik mermaid sequence diagram is adjusted to reflect more accurately the actual supported timeouts.

Which issue(s) this PR fixes:

Does this PR introduce a user-facing change?:

Align documentation about HTTPRouteTimeouts.BackendRequest timeout scope.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

Hi @rtribotte. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[youngnick]

/ok-to-test

This seems reasonable, but I'll need to have a think about it in case it counts as a breaking change.