Align GRPCRoute hostname cross-serving godoc with the relaxed site docs

[lexfrei]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

#4598 relaxed the cross-serving rule on the site page: implementations may enforce hostname uniqueness between GRPCRoute and HTTPRoute, they no longer must. The GRPCRouteSpec.Hostnames godoc still says MUST, and that text is what ends up in the CRD descriptions and the API reference. This changes the godoc to match and regenerates.

Left alone on purpose: GEP-1016 still carries the old wording, and the "When to Use GRPCRoute" section of the site page still says to use HTTPRoute for both when sharing a hostname. I can fold either in if you want them here.

This PR was written in part with the assistance of generative AI.

Which issue(s) this PR fixes:

None

Does this PR introduce a user-facing change?:

NONE

AI disclosure

This PR was prepared with AIL:3. I personally reviewed each line of the submission prior to opening this PR.

[kubernetes-prow]

Hi @lexfrei. Thanks for your PR.

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: lexfrei
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[rikatz]

/ok-to-test

[lexfrei]

This is a two-line godoc alignment with the site docs; CI is green since the ok-to-test. Anything needed from my side?

[youngnick]

Yeah, I didn't know about #4598, but this definitely does what that did. So, I guess final approval is for @rikatz when he is back from leave.

/lgtm