cleanup windows install docs

What does this MR do?

• Add missing steps to Windows install details

• Use gitlab-runner rather than gitlab-ci-multi-runner.exe in support of https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1195

• Document how to use with true service account (Built-in System Account)

• General cleanup

• Provide better link on how to run elevated command prompt

Replaces https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/482 & https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/492