Document how to specify environment variables during manual pipeline run and more
Documents:
- How to run a manual pipeline.
- That manually specified variables are available.
In the edited files, I've also tried to make the documentation adhere to the style guide more.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/46364.
Required porting: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7260.
Edited by Evan Read