Update ReferenceGrant docs to include Gateway -> Secret use case

[nathancoleman]

What type of PR is this?

/kind documentation

What this PR does / why we need it:
The docs for ReferenceGrant in some cases only cover the route -> backend use case where they should also include the gateway -> secret use case for certificate references.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

Update ReferenceGrant docs to include Gateways that reference a Secret in a different namespace

[k8s-ci-robot]

Hi @nathancoleman. Thanks for your PR.

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[youngnick]

Nice catch @nathancoleman!

/approved
I'll leave the LGTM for a second review.

[shaneutt]

/lgtm

[nathancoleman]

/assign @bowei

[robscott]

Thanks for the work on this @nathancoleman! I agree with the goal here, but I think it may require a bit more restructuring.

[robscott]

I think we need some more nuance here. I'm not really sure how this will scale since we really need to document each valid pair, ie HTTPRoute -> Secret doesn't make sense, and there's not currently any way to do Gateway -> Service. I think the best option here is to add a note here that ReferencePolicy is only supported as explicitly documented on object reference fields. Then we can add a comment on the cert ref namespace field in Gateways that references to Secrets in other namespaces can be allowed by ReferenceGrant. Similarly, we can update the BackendRef namespace field to state that references to Services in other namespaces can be allowed by ReferenceGrant.

[mikemorris]

The status Condition -> Reason mappings (see example in #1114) may provide a pattern for clarifying these relationships a bit.

[nathancoleman]

@robscott @mikemorris thoughts on the most recent commit?

I'm having trouble regenerating the CRDs on my machine, so those aren't updated in this PR yet.

[robscott]

/retest

[robscott]

Thanks @nathancoleman!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nathancoleman, robscott, shaneutt

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