Mention auto-create on event registry doc

[matzew]

Proposed Changes

• adding hint about auto create on event registry doc

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[dsimansk]

/approve
/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dsimansk, matzew

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[matzew]

/cherry-pick release-1.13

[knative-prow-robot]

@matzew: new pull request created: #5845

In response to this:

/cherry-pick release-1.13

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