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.

@gl-docsteam .