docs(api): 📝 Fix HPA docs with addition of "External" type

[ghostsquad]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Documentation did not accurately describe the valid values of Type for HPAs.

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:
N/A

Does this PR introduce a user-facing change?:
N/A

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:
N/A

NONE

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[k8s-ci-robot]

Hi @ghostsquad. 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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[ghostsquad]

/assign @lavalamp

[fejta-bot]

This PR may require API review.

If so, when the changes are ready, complete the pre-review checklist and request an API review.

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[krmayankk]

/ok-to-test

[krmayankk]

/sig api-machinery

[krmayankk]

/lgtm

[lavalamp]

You will need to regenerate all the stuff. hack/update-generated-all.sh or something like that.

[ghostsquad]

/retest

[ghostsquad]

/assign @lavalamp

[lavalamp]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ghostsquad, lavalamp

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Needs approval from an approver in each of these files:

• api/OWNERS [lavalamp]
• staging/src/k8s.io/api/OWNERS [lavalamp]

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[ghostsquad]

@lavalamp what's next here? there's still a do-not-merge label, and I'm not sure what I need to do.