Docs: improve clarity and security guidance for SSH keys in CI/CD

I prepared a small documentation-only change to improve clarity, consistency, and security guidance in the SSH keys with GitLab CI/CD page.

Unfortunately, I’m unable to push a branch or open a merge request because my fork repeatedly exceeds GitLab’s storage quota due to the size of gitlab-org/gitlab, even after re-forking and using the Web IDE.

I’m attaching the patch below for maintainers to apply.