Provide more details on route parentRef in documentation for routes <> Gateways attachment.

[cxhiano]

What type of PR is this?
/kind documentation

What this PR does / why we need it:

gep-957 proposed to add Port to ParentRef, in addition to existing fields. This PR updates existing route attachment documentation to provide more details on parentRef,

Fixes #1021

Does this PR introduce a user-facing change?:
NONE

[k8s-ci-robot]

Hi @cxhiano. 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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[robscott]

Thanks for the work on this @cxhiano! Mostly LGTM, just a few small comments.

[robscott]

/ok-to-test
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cxhiano, robscott

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[youngnick]

Yep, this looks great, thanks @cxhiano!

/lgtm