Refactor opening paragraph in admin docs

What does this MR do?

Refreshes the open paragraph in the admin docs section, just to make it more parseable and less duplicative.

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9587

Review checklist

• Technical writer's review (required)
• Merge the EE-MR first (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9587), CE-MR afterwards