fixes incorrect docs on ingressClass enabled by default

[longwuyuan]

What this PR does / why we need it:

The faq for migration to ingress api v1 was hurriedly prepared.
This PR will try to improve on the faq with more content or altering content.
This is WIP

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

fixes #7591
fixes #7539

How Has This Been Tested?

Issues, chats, docs, blogs, discussions, suggestions etc

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]

@longwuyuan: 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.

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[rikatz]

/lgtm
/approve
Thanks

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [rikatz]

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