Document the list of places where you are allowed to use an env variable in .gitlab-ci.yml

From https://gitlab.com/gitlab-org/gitlab-ce/issues/38136#note_41037803

some sections don't allow variables, there should be an issue to at least document this

So, the full list where you are allowed to use an env variable is unknown, and it should be looked in the code or by trial and errors, or preferably both.

Eric tested this a bit as you can see form this comment https://gitlab.com/gitlab-org/gitlab-ce/issues/38136#note_41354724