Update annotations.md - improvements to the documentation of Client Certificate Authentication

[daniel-kun]

Improvements to the documentation of Client Certificate Authentication. (auth-tls-* annotations).

• Mention that these rules are applied per host and not per Ingress/path
• Include more possible and default values
• Describe the headers that are sent to the upstream services

What this PR does / why we need it:

The docs on the aut-tls-* annotations was missing a few important details, which I added.

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)
  (Only documentation)

How Has This Been Tested?

I proof-read the changes and did a few local experiments with the annotations to verify the described behaviour.

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.

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[k8s-ci-robot]

Hi @daniel-kun. Thanks for your PR.

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[aledbf]

/ok-to-test

[aledbf]

/lgtm
/approve

[aledbf]

@daniel-kun thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, daniel-kun

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