Add some extra detail to the client cert auth example regarding potential gotcha

[SgtCoDFish]

What this PR does / why we need it: I'm directly aware of at least two cases (one of them being myself!) where somebody was caught out by the fact that the trusted client cert issuers for an ingress resource must be given in a file named ca.crt and that other filenames will fail to work entirely. Given that it's happened at least twice, I'm assuming it's something that could catch more people out and is worth mentioning in the client certificate example documentation.

This change makes it clear to those reading the documentation that this is a potential gotcha. Also, while I was at it, I improved some of the language in the example to be slightly clearer.

Which issue this PR fixes: N/a, documentation improvement only.

Special notes for your reviewer: N/a as far as I can tell. I just signed the CLA - I hope I did it correctly!

[k8s-ci-robot]

Thanks for your pull request. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please follow instructions at https://git.k8s.io/community/CLA.md#the-contributor-license-agreement to sign the CLA.

It may take a couple minutes for the CLA signature to be fully registered; after that, please reply here with a new comment and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please sign in with your organization's credentials at https://identity.linuxfoundation.org/projects/cncf to be authorized.
• If you have done the above and are still having issues with the CLA being reported as unsigned, please email the CNCF helpdesk: [email protected]

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[SgtCoDFish]

/check-cla

[aledbf]

/lgtm
/approve

[aledbf]

@SgtCoDFish thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [aledbf]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment