Document four backticks feature in markdown

Evan Readrequested to merge
docs/guidance-for-four-backticks into master

What does this MR do?

Now that we make use of the four backticks feature, we should document it.

Related issues

Relates to: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22249 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24321.

Author's checklist

Review checklist

  • Technical writer's review (required)
Edited by Evan Read

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