Improve docs about certificates when enabling TLS for Gitaly nodes

What does this MR do?

The documentation about configuration of Gitaly to use TLS is hard to read with regards to where Gitaly's certificates need to be installed to. While the documentation already mentions that certificates need to be installed into /etc/gitlab/trusted-certs, the paragraph mentioning it is formulated in a confusing way. This MR rewords it to hopefully make it easier to understand.

Related issues

References gitaly#2671 (closed)

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• Apply the documentation label, plus:
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  • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
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• Link docs to and from the higher-level index page, plus other related docs where helpful.
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• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

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1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
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2. Ensure a release milestone is set.
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