docs: Clarify default-backend behavior

[sandipb]

Clarify default-backend behaviour for services with multiple ports.

Also minor fixes for typos and language consistency.

What this PR does / why we need it:

The nginx.ingress.kubernetes.io/default-backend annotation does not clearly mention how the port selection of service is done, especially if the service has multiple ports. This caused a lot of frustration for me recently when I was attempting to use this annotation. I hope others are not bitten by this.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Which issue/s this PR fixes

N/A

How Has This Been Tested?

N/A

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added tests to cover my changes.
• All new and existing tests passed.

[k8s-ci-robot]

@sandipb: This issue is currently awaiting triage.

If Ingress contributors determines this is a relevant issue, they will accept it by applying the triage/accepted label and provide further guidance.

The triage/accepted label can be added by org members by writing /triage accepted in a comment.

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[k8s-ci-robot]

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[k8s-ci-robot]

Hi @sandipb. Thanks for your PR.

I'm waiting for a kubernetes 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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[rikatz]

/lgtm
/approve
Thanks

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rikatz, sandipb

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• OWNERS [rikatz]

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