Document GIT_STRATEGY=none

## What does this MR do?

Adds documentation for the new `GIT_STRATEGY=none` option in GitLab Runner v1.7

## Are there points in the code the reviewer needs to double check?

Speling and grammar, of course.

I'm using the term 'project workspace' to refer to the build directory, which is a Jenkins-ism. Is there preferred alternative wording?

## Why was this MR needed?

Discoverability of new feature

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- <strike>[ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added</strike>
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- <strike>[ ] API support added</strike>
- <strike>Tests</strike>
  - <strike>[ ] Added for this feature/bug</strike>
  - <strike>[ ] All builds are passing</strike>
- <strike>[ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)</strike>
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #21337 

Depends on http://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/332

See merge request !6577