Add mention of secure-backends to backend-protocol docs

[carlosdp]

What this PR does / why we need it:
This PR adds a mention of the deprecated secure-backends annotation to the new backend-protocol annotation. I spent all day trying to figure out how to connect to a secure backend and only finally figured it out because of #3416 .This PR adds a little note that makes it easy to Ctrl-F the old field and find the new one since many articles / SO answers reference the old secure-backends.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #3416

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[carlosdp]

signed CLA

[aledbf]

/check-cla

[aledbf]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, carlosdp

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[carlosdp]

my bad, it hadn't gone through, now CA is signed