Add documentation about secure-verify-ca-secret

[maxlaverse]

The annotation nginx.ingress.kubernetes.io/secure-verify-ca-secret was introduced (#716) without any documentation.

This adds a few sentences about it.

[maxlaverse]

/assign @nicksardo

[codecov-io]

Codecov Report

Merging #2169 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2169   +/-   ##
=======================================
  Coverage   36.45%   36.45%           
=======================================
  Files          69       69           
  Lines        4861     4861           
=======================================
  Hits         1772     1772           
  Misses       2819     2819           
  Partials      270      270

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[aledbf]

/approve

[aledbf]

@maxlaverse thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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