Update GitLab docs for API token usage

Review Request #9809 — Created March 19, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x
cdd72b6...

Reviewers

The GitLab hosting service now requires providing an API token directly
so we can communicate with API versions 3 and 4. This patch updates our
manual with this information.

Built the docs and looked through them.